Calculating ROI for an ASP project.

I am trying to calculate the ROI for a project where I am implementing an ASP provider that calculates their fees based on the number of applications that complete using their system.
(i.e. $10.00 per application so 1000 apps/month would be $10,000.00 / month fee)

Can anyone help? Is there any templates or files that I could download or have emailed.

Thanks