Just adding my two cents on this. What you are proposing is very similar to the way I am making my scout trooper costime. Pep, harden, sand and vacuform (in theory that is. I have made most of the armor pieces, and most of the vacuform machine. still need to find parts for the heating element.) Yes, it will be more expensive than buying a kit. But at the end of the day, it will be made by my hand, and that is my point in doing all of this, and sounds like yours too.
In regards to the cost in general, seeing as the other ways to make your own molds to vacuform include either and ungodly ammount of modeling clay or specilized wood carving tools, the cost of bondo/card/fiberglass resin/fiberglass mat doing it pepakura will be somewhat cheaper, if you buy in bulk.
The Queen of Hand Guards is right about one thing. It will take you and ungodly ammount of time. But you are going to be so satified with each piece you build.