Calculating valuation when using web-service back-end?

All,

For the YC funding application, I'm having difficulty determining valuation. The reason being that the back-end would use web-services for computing/storage (yeah, they do more than sell books :-)

Should I do an estimate based on an ideal system running at 100% processing usage? This may seem appropriate for a system that produces widgets, but mine mainly deals with running queries.

Thank you for your help! NoMoreSnow