I'm trying to find a tool that will allow me to calculate the true cost of fund expenses. Not only how much I will be charged in expenses but also how much money those lost expenses could have made if they were reinvested. Does anyone know of a good tool?

Alternatively, is this a correct approach:

I'm going to assume 7% rate of return.

Fund one has an expense ratio of .5%

Fund two has an expense ration of .08%

Can I just use one of the many compounding interest calculators on the web and plug in a 6.5% rate of return for fund one (7-.5) and a 6.92% (7-.08) rate of return for fund two and then compare the difference? Math is not my forte so I'm not sure this is mathematically correct.

I was going to use this calculator:

http://www.moneychimp.com/calculator/compound_interest_calculator.htm