You can exponentially increment the price (ex. x1.25 everytime)

So this would give a sequence of prices like follow (assuming the base price is 10): 10, 10x1.25, 10x1.25x1.25, 10x1.25x1.25x1.25, etc.

=> 10, 12.5, 15.625, 19.53...

(Obviously you should round these values so the actual price would be something like 10, 13, 16, 20, etc..)

You can change the ratio if you want the price to grow faster