What I want to do is fairly simple. I just want to send a user who has signed up for one of our fee-based webinars to authorize.net to handle the payment processing and then come back to our site and dump the registration info and payment confirmation into sfdc.

Now, on a non-Sites website I could do this fairly easily in php, java, or any of the other apis authorize.net provides. I would just have a form to collect the basic registration information, send that to authorize.net, get the confirmation back and drop the info into a db.

However since Sites is not using (not that I can access) php, java, etc. and they don't have an apex or a javascript api as far as I know, anybody have any suggestions how to interface to authorize.net? 

Please do not offer appExchange solutions as they are not in our budget right now. 

Thanks.