Like ecommando said, don't get sucked into it though, unless you can control every aspect of the deal, code, database, etc... As the white-label grows they will expect the world from you in terms of support/updates/code modifications/etc... They will take away from your time to support your own customers. I would offer a hybrid solution at most where you allow them to change the logo and remove references to your company, but you continue to host and control the code.
To answer your question on why they wouldn't pay/use, well it is all about marketing. They want to be able to offer something different from their competitors and make it look like they are the ones behind it.
As for pricing, NEVER, NEVER take a lump sum payment. Always work something from a revenue-share or monthly fee arrangement. You could even do an annual contract with built-in price increase each year based on percentages of their sales/revenue.
If your product is that good that other companies are approaching you to white-label the solution, the best advice I can give you is to review your marketing/sales strategy. Why can't you sell direct to their market? Why risk losing revenue and allowing them to make money off your product?