Yes try Google docs spreadsheet. It is the only possible answer for this question.
My reading of the OP suggested that the spreadsheet was to be used as a form of commercial quote engine. The user is offered an input form on which he/she supplies some parameters, and the spreadsheet (or whatever) works in the background to produce the response.
If that is what was intended, does this spreadsheet have the facility to work transparently like that? If so, is there a charge for using it commercially?
Also it seems a little premature to rule out a server-side app written in PHP or whatever. In the past, I've used a spreadsheet to mock up an app then coded it once I had got the logic right. That wasn't a web app, but the principle applies.