VoiceXML has the exact same processing rules as HTML. You are generating a page via ASP. Once that page is generated you have a static file that is provided to our IVR system. The VoiceXML programming langauge does provide you with two means to accomplish what you are looking for. You can either take the collected data and submit it to a new ASP page (like an HTML form), or you can attempt to perform the calculation on the data using the built-in JavaScript interpreter.
Using VoiceXML should not be very different from HTML, you are collecting information from the user that is stored in local values on the browser. Those values are submitted as text strings your IVR application server which you use to generate a new page.