A group of monsters roaming around the outside of your world stumbles across a "dungeon" that is completely empty of treasure-hoarding creatures. Oh, there are probably beasties there already, but let's say nothing intelligent. If you like, this is hundreds and hundreds of years ago, long before your party shows up to plunder the place.
Let's further imagine that these monsters are carrying only 100 g.p. with them. We can envision them as a few orcs, goblins, whatever you like - but not some enormous band. They settle down into the top level of the dungeon, near the surface, and for a year they do little or nothing.
Now, these creatures don't have much purpose for the coin they have; they're not going out to the local market or anything. But they like the coin, its shiny and it has pretty pictures on it. They give it to their children to play with. And now and then, some of it is dropped, some of it is stolen by the little beasties in the dungeon, and so on. Call it 'spoilage.'
At the same time, these creatures are occasionally going out into the world and snatching more treasure from other things, but not much - just enough to say that in a year, they steal about 100 g.p. worth of hard, valuable, endurable treasure.
Now, I'm a great fan of the 1% per month maintenance rule in the DMG, and I've argued in the past that this could be read as general loss. So for the purpose of our proposal, let's say that 12% of whatever treasure our 1st Level dungeon creatures have is lost ... it drifts down deeper into the caverns, where it is picked up by gelatinous cubes, wandering troglodytes, giant rats and so on.
Suppose that once every three years, a group of somebodies - a party, a local constabulary, what you will - comes along and clears out the upper level of the dungeon, taking all the treasure that is there. And still, the caves are comfortable, so let's say that something else moves in, and in the process brings in some of a hundred gold pieces, depending on how long they've been there that first year. We can roll a hundred sided die to see how much precisely. And the new creatures go on accumulating a hundred gold a year. For centuries.
And let's add that of whatever drifts into the 2nd Level, 12% of that drifts into the 3rd Level ... and 12% of that drifts into the 4th Level, and so on. Only the 2nd Level is only plundered 1 in 9 years. The 3rd Level, only 1 in 27. And the 4th Level, only 1 in 81. And so on.
Now what would that look like?
Here's what you do.
On Row A of your excel sheet, in Box B1, Write "Year".
In boxes A3 going down, write "Level 1" "Level 2" "Level 3" and so on.
In box B2, write "1"
In box B3, write "100"
In box B4 going down, write "0" for as many levels as you want.
In box C3 going down, write "=RANDBETWEEN(1,3)". You might want to make this column a different color, so that you know these are random numbers and NOT your results.
In box D2 write "=B2+1". This will serve to duplicate itself and count the years for you.
In box D3 write "=IF(C3=1,RANDBETWEEN(1,100),(B3*0.88)+100)". All that says is that if the random number generated is a 1, the first level of the dungeon was cleaned out. If it wasn't cleaned out, then 12% is gone (spoilage) and the rest is added to the 100 g.p. the creatures on that level gathered that year. It also says that if the dungeon was plundered, 1 to 100 g.p. is accumulated in on the first level AFTER the plundering.
In box D4 write "=IF(C4+C3=2,0,(B3*0.12)+(B4*0.88))". Which says that if the dungeon above was cleared out, and the roll for this level was also a 1, then this level was cleaned out too. Spoilage from Level 1 pours into Level 2, and we minus the spoilage from Level 2 into Level 3.
In box D5 write "=IF(SUM(C3:C5)=3,0,(B4*0.12)+(B5*0.88))". Level 3 is only cleaned out if the two levels above it are, so the sum of all three random numbers must be 1 x3 = 3.
In box D6 write "=IF(SUM(C3:C6)=4,0,(B5*0.12)+(B6*0.88))"
In box D7 write "=IF(SUM(C3:C7)=5,0,(B6*0.12)+(B7*0.88))"
And so on, remembering to adjust the sum for each number.
Now, you can copy everything in column C and D into columns E and F. That will give you the results for year 3.
In fact, you can now copy as far into the future as you wish. The numbers get quite interesting as you get past 80 years or so.
If you've done it right, you should have something that looks like this:
I've colored the random numbers in coral, so that the actual results in g.p. are in white.
Yes, I know, it does not look very impressive for Level 5. What is particularly interesting about this generator is that no matter how you fool with the numbers, you're going to find your dungeon pouring out its gold into the bottom. You have to define one of the levels as THE bottom level ... and that one level will pile up gold impressively ... but none of the ones between the top and the bottom will.
You can reduce the number of times a level is plundered, but that only makes it drain more gold through. You can reduce the amount of "spoilage" ... but that only makes it harder for the bottom levels to accumulate gold. Reducing the likelihood of plunder down to 1 in 6, and decreasing the spoilage to only 2%, it generally took more than a hundred years of dungeon just to get the gold on the 5th Level up to 100.
In other words, I'm saying that if anyone else in the world is plundering dungeons, the idea that the space between the top of the dungeon and the bottom are filled with treasure just doesn't fit mathematically. And if no one is plundering these dungeons, then they must be very remote - which begs the question, where are they getting the gold from?
Please, don't argue that the people below are robbing the people above - that just makes the dungeon sieve move faster.
I'm not saying I'd run a dungeon differently ... but one has to recognize that, economically, the process CAN'T work like you think it works.