As of this time (Jun-2012), iis not possible to use formulas within Google Form ...

however, there may be some workarounds to meet your intent -- for example, 

let us say you have a web page via which a potential respondents submits a Form -- 

the needed calculation is performed in the Form Responses sheet of the associated spreadsheet, 

you can set up your spreadsheet in such a way as to publish a range of cells showing the form submitter's ID and her/his calculated amount 

so this way, after the person submits the Form, she/he can read on the same web page the amount owed (within a few minutes) on refreshing the browser

I hope this helps.