My suggestion would be to use a more complicated formula, such as rand((1 - exp(-#CASHPOINTS))*50, 100) > 49
So with 0 points the probability to win is 50%. It will increase as the player gets more points, but only approach to 100%.
So it will be better to make a script command to do the calculation in float, use a number (POINTS) as input and return success or fail.