How to enforce cap/limit on the commission amount?

Hi, 

I am stuck in a problem. I have a commission type in which I have to put a limit on the commission amount. Following are the details:

Suppose that the limit of the compensation amount is $200. 

Now what I need is that if my transaction amount is greater than equal to $50 and less than equal to 200 ( 50 <= TRANSACTION_AMOUNT <= 200 ), then the commission amount should be 100% of the transaction amount i.e. if T.Amount is $150 then the commission will be $150, similarly if the T.Amount is 200, the commission amount will be $200. 

However if the T.Amount is $500, the commission amount should still be $200. How can this be achieved?