What about a system like this. When someone has an opurtunity to place a bounty they may set up a bounty 1 time only for any amount that they can imediately afford and that amount is then put into escrow. But what the owner of the bounty can also set up is how much money is given out each time (with a hard minimum to avoid 1gp or similar amounts to prolong the effects of the bounty).
So if a 1000gp bounty is set to be paid in increments of 100gp per kill then it will last 10 kills. What this would do is still allow wealthy players to create a greater incentive for others to exact retribution but they are atleast restricted by what they can afford at that moment instead of using future wealth to keep a bounty running indefinitely.
Or you could also make the bounty have to be paid in a set amount of increments (like 500gp, something significant but also affordable) and only give the owner of the bounty controll over how many times that can be paid out based on how much they are willing to and can afford to invest at that moment.