Formula for Pro Rata Multiplication

Hi,

I'm looking for a formula that will multiply pro rata. I can manage my way around a formula when it is written but when i have to start from scratch i get stuck in difficulties.

Here's the criteria:
If A1 is less than 150 then answer is zero.
If A1 is 150 to 200 then divide A1 by 200 x cost
If A1 is greater than 150 then A1 equals cost.

basically, under 150, we don't pay so answer is zero, if between 150 - 200 we pay pro rata cost and over 200 we pay full cost.

I can't get there myself with formual, anyone help?

Thanks