Licensing fees for embedded files

My company created a website with a unique grid that displays lots of vendor pricing and other data all in one place. It's a one-stop-shop for any Optometrist that is buying an Electronic Medical Record.

Link

A larger site that also houses information and forums pertaining medical doctors has contacted us to 'license' our grid for use on their site. They have almost 4 million unique hits over 3 years. This grid would be a significant addition to their site.

Does anyone know what we should charge for this? Should it be based on hits or just a flat monthly fee? We have a fee set in mind, but we don't want to lowball ourselves.