I remember running the numbers, as a math geek, in high school and I believe once you are within a $200 range of the target, you stand a constant one in thirty chance, assuming the riddles are worth $50 and $200, on each call for a new riddle. Once over, there is zero chance until you come within a $200 range again. The odds to not get better or worse as you get closer, as random numbers have no memory.

Someone with better skills is more than welcome to check my math, and the formula I'm using.