I'd have the difficulty ramp up way slower over a good number of levels, like, a tonne of levels. Have it so you can buy items with points as usual, or real money. No sending you back to the start of the game on game over, have say 3 levels then a boss and after the boss it's a check point... SO you die on a boss and you're either sent back 3 levels or you buy an extra life for 99p. Some people will pay and people like me will go back and replay 3 levels until I beat it, since that's what I do on Fantasy Zone anyway, and love it.