Check out this site. It discusses foam bending and a whole lot of other building techniques using household/hardware store stuff as opposed to surf industry materials.
There are also lotsa videos on the youtube showing board building techniques.
You can do it, but you're making a lot of extra work for yourself to save money. But if you have time and no money, it can be a fun project and you'll learn a LOT.
Bend in your rocker when you glue up your foam using a rocker table. XPS can be tricky and prone to delam. Do your research and read up. Cut your losses by learning from the experiences of others.
You'll get great satisfaction from it when you pull into that first double-up dredger and come out smiling.
First time builds are always fun...you learn plenty and have a ton of fun. Whenever people are interested in a first time build and going route you are headed...I always reference the following thread from swaylocks.com (BTW...you should be reading as much as you can on that site...it wil help). Here is the link: http://www2.swaylocks.com/node/1020650.
This build would provide a lot of perspective for you in light of the direction you are going...the rest you'll learn by experience. Have fun!
i based my entire board off that swaylocks thread. hoping to make another board this summer.
what i learned? a lot. mainly to not try and make a shortboard.