Using $_GET for coupon codes. Show pre-discount price with a strike-through.

Hi there.

I have a site that is using the auth.net plug-in to do some ticket sales handling. I added a $_GET['ccode'] to determine if a person has a valid coupon code or not. Originally, I was just gform_pre_render the form based on whether this $_GET value was set or not. This can be seen here http://pastebin.com/z2yrXUcW. isValidCCode simply checks the $_GET and checks it vs valid codes and returns true or false.

Now, the problem I'm having is. My client requested me to display the ORIGINAL price on the form, with a strike through, and then the MODIFIED price afterwards. What would be the best way to go about doing this? Which hook affects the actual HTML output of the form fields (if any)? Would there be a better way to go about coupon codes?

Thanks!