"Dear old gent passing by
Something nice takes his eye,
Everything's clear, attack the rear
Get in and pick a pocket or two.
When I see someone rich
Both my thumbs start to itch
Only to find some peace of mind
We have to pick a pocket or two."
-- Oliver, the Musical
I have never been a fan of the pick pockets ability. I quite like it when a player is clever with it, uses it to filch a map off the bar from two lads planning a robbery, or kypes a dagger from a fellow's belt - that's sweet when it happens. But what I usually hear - from people who have played WAY too much D&D in old style games - is the cliche:
"I look around the market: is there anyone who looks like they're carrying a fat purse?"
Jeezus. Can you feel your eyes roll?
The answer to that question is, unfortunately, yes. It's a market. People bring their money. What's more, other thieves than the player know this. Some Master Thief knows this, and that's why he or she has the boys pick a few here and there without spoiling the field by taking too much! That's why there's an understood rule in the town underground that this particular market square is OWNED by Ricktus the Unforgiving, who cheerfully murders dumb-ass outsiders who think they can saunter onto his stealing grounds and pluck plums he's let dangle for months. The last thing Ricktus wants is for some tight-ass town official with tendencies to blood vendetta to have his pocket picked ... but of course the fucking player doesn't think about these things, because the fucking player has been playing for 15 years with fucking DMs who don't think about these things. So then I have to kill the player's thief, and smartly, because even if the official doesn't notice it, the 4-24 other thieves who are always in the square see the player's thief pick that pocket plain as fucking day.
This tends to produce two effects: 1) the player cries and whines and never plays in my world again; and 2) the player never uses their thieves' ability to pick pockets.
My feeling is, (1) good riddance; and (2) if you want to use it, think it through.
Since that isn't working, however (because players can't be bothered for a few coins), and since I'm rewriting the thieving abilities anyway, I will relax my position on the matter and see if I can't arrange a compromise that retains the master thief's status and enables the player to reasonably secure a few coins.
As ever, the first real problem is the percentage. It doesn't convey any measure of whose pocket is being picked, so that the 1 HD prostitute is as easily picked as the 1 HD village idiot. You can modify for level, but there's no modifier for streetwise (and no measure, either).
The second problem is that of identifying the target. Let's ask the question, is the actual target of the theft important? If the player is caught, then the player is going to have to run from virtually every authority in the square no matter who is the target. Remember, this isn't a poor production of Oliver, where the thief runs ignored through the streets, pursued by the gentleman who is ignored. This is a square with a potential lot of spellcasters. You really want to mess with your thief as a DM? Have a tree suddenly appear two feet in front of the thief just as the thief looks over his or her shoulder - they turn back, there's a tree, no time for disbelief - BANG! One unconscious thief. Whereupon the first level illusionist with phantasmal force walks up to the gentleman and says, "Half?"
Think of the spells: magic missile, grease, hold person, push, trip, blind ... and locate object, augury, divination, etc. Realistically, in the age of magic, with so many easily applied spells available to first level casters, once the thief is revealed, that thief is caught. This is usually overlooked by DMs (who haven't the imagination Ron Howard gave a Pie) ... but I don't overlook it. If you want better, smarter players, you shouldn't cater to their laziness by overlooking it, either.
Let's say we don't need to identify the target. Let's further say there doesn't have to be one target. After all, what the player actually wants is the money. There's a rule floating around that picking pockets is worthy of gaining experience, but seriously, I hope DMs are not dumb enough to follow such rules. If they are, well, what I'm about to suggest won't change it.
If the target is unimportant, we can perceive the market square (or the whole town or village) as a kind of field, which the thief picks from. If we suspend the troublesome percentage role (yes, "role," not "roll" - I have ideas for the %), we could employ a different measure for the thief's efforts. A measure like, How Much?
It's reasonable that a high level thief ought to be able to take more from that field than one that's lower level. If we impose a rule that says a thief must spend one full day in examining the field, identifying other thieves who may be watching, picking their moment and choosing from opportunities (assumed, not described by the DM) as best they can, at the end of the day the 8th level thief will have more to show from that work than the 1st level.
In other words, we can roll dice to determine how much is taken, not IF something is taken ... so that the 1st level can take a day to get money for the inn that night, and an 8th level can help fund the party's latest adventure by lifting more.
Mm, but how much more?
It has to be little enough so that it doesn't unbalance the game in favor of the thieves' take. If the thief is lifting 200 g.p. a day by 6th level, well, I think my thief is going to take a year and visit London. At the same time, it has to be enough to count for something. I propose a sort of lottery.
Let us say that the base theft for a day is 4d4+4 c.p. That's a range of 8-20. That's not much, but it's a bit of pocket change for a low level thief.
We can then modify this by multiplying it against an increasing modifier. For such things, I love the Fibonacci series: 1, 1, 2, 3, 5, 8, 13, 21, etc. Each number is equal to the sum of the previous two numbers, so that the next number in the series would be 13+21 = 34. This series doesn't climb as quickly as exponents of 2 (1, 2, 4, 8, 16 ...) but it does climb meaningfully. Thus, the 8th level thief is lifting between 168 and 420 c.p. per day (or 1 to 2 gold, depending on how you want to roll).
This isn't much, and shouldn't be much ... too much, like I said, and they'll leave off adventuring.
Yet I have an interesting way in which this can be enhanced. Let's say that for one day's stealing, there's a low chance (1 in 20) that the thief will get lucky. They'll pick up 10-100 g.p. in one pull. Let's further say that the thief gets 1d20 roll per level to see if they're lucky. This means that, even though the 2nd level pulls as much as the 1st level (note both the first numbers in the Fibonacci series are '1') ... the 2nd level gets twice the chance to get lucky.
Let's further say that multiple 1s on d20s do NOT mean multiples of 10 to 100 gold in one pouch ... but that every collected 1 can be rolled again, so that on the second round the hit might be as much as 100 to 1,000.
For example, suppose that Dorry the Doorslipper is 6th level, and she spends the day thieving. She throws dice, getting a 13, multiplying that by 8 (see the series) and getting 104 c.p. worth in silver and copper (how you determine that is your business). Dorry then throws 6d20 for her luck ... she rolls two 1s, and so gets the 10 to 100 g.p. bonus. She then rolls both 1s (the other four dice are ignored) to see if she does even better.
You can decide if you want to allow a 1,000 to 10,000 g.p. option. (It's a 1 in 8000 chance, so the payoff is slightly worse than 1:1 - 11:16)
Ay, but here's the rub: if the player wants the piddling few coppers, they can take them without risking any chance that the local thieves' guild, or any other witness, will see them. But after, repeat AFTER, the player rolls a 1, so that they SEE the plump pouch, they then have to decide if they want to throw the % listed in the player's handbook to take that pouch.
Worse, if it is a VERY plump pouch, the 100 to 1,000 g.p. pouch, they have to roll that % twice to get it. This is because that pouch (or tiara or jeweled sword or whatever you decide it is) is more obvious to everyone in the square ... there are more people watching it. Thus, if Dorry gets a second 1 in a row, and sees that very tempting piece, will she dare take it? Only Dorry will know.
If she is spotted, well ... as I said: spells, spells, spells, and of course a very angry guard and or master thief.