I don't think it would be very popular. Assuming you charged per run, there is a good chance people will end up with very little reward (unless they bought TONS of runs). It is much easier to pay for a more conventional money making method.
Perhaps if you did something as offering a certain price and that would entail you run it until they get one item, then offer another price for two items, and onward. That may work. Then it is still a gamble with what you would get, but people may be more willing.