But there is a certain school of thought that cries, "Heroes don't carry change! I don't even carry change!" and so abstract wealth systems exist.
But I haven't found one that is based on true scientific principles, and so here's my contribution.
Think of it as a series of coins, each one worth twice as much as the last one. Also, they're so big that you can only carry one at a time. You just carry around a Wealth Rating in your moneybags, instead of 900 gold coins. And things that you buy or steal also have Wealth Ratings. Maybe Worth Rating is more accurate.
So, assume the system is calibrated thus:
5 gold = 1 Wealth
10 gold = 2 Wealth
20 gold = 3 Wealth
40 gold = 4 Wealth
80 gold = 5 Wealth
etc. . .
So, if I have 4 Wealth and want to buy something that costs 3 Wealth, I have exactly 3 Wealth remaining after I buy it.
If I have 4 Wealth and want to buy something that costs 2 wealth, that drops me halfway down to 3 Wealth, but since there is no halfway in this system, there is instead a 50% chance that it drops me down to 3 Wealth.
Therefore, if I have 4 Wealth and buy something that costs 1 wealth, I have a 25% chance to drop to 3 Wealth.
And so on. All the probabilities are going to be exponents of 2, so just roll some coins.
Adding is equally simple.
10 Wealth + 10 Wealth = 11 Wealth
4 Wealth + 4 Wealth = 5 Wealth
4 Wealth + 3 Wealth = 50% chance of increase to 5 Wealth
4 Weath + 2 Wealth = 25% chance of increase to 5 Wealth
10 Wealth + 8 Wealth = 25% chance of increase to 11 Wealth.
You probably should cap it at "no more than 3 coins rolled at any one time". And don't get too caught up in the math. The whole point is to streamline the process. If the PCs want to buy a bunch of shit at once, just eyeball it and say "about 10 Wealth, or 9 if you'll settle for donkeys instead of horses." And it works just fine whether you are dealing with Wealth 12 (about 10,000 gold) or Wealth -5 (about 8 silver).