You can use form values to do this... although this is not all that secure.. since anyone could essentially change the HTML and submit that instead of the actual calculations..
Your best bet is to store your price's in a database and only pass the ID values of the items to your SSL, which in turn could grab the values and process them on the SSL side of things and submit the values..
I am still unclear as to why you would get anyone to enter purchase information on a non-secure page and only pass them to the secure site for the CC Gateway... the CC Gateway doesn't need the SSL website to do its thing..
Maybe its just late and I haven't understood your question very well..
Anyhow, I hope this has helped,
Cheers.
Gorkem.