Your cheapest option is to use a very basic derailleur with an integrated hanger:
http://www.nashbar.com/profile.cfm?c...%20Derailleurs
you will need to cold set the rear triangle (or whatever its called) and rout the shifter cable over the top tube (doable with clamp-on cable stops) and then rig up a couple of stops on the seat stays (pipe clamps?)

you probably will want to use only one ring on the front so you'll have to find a way to keep the chain from jumping off. You can use a guard in place of the outside ring and a jump stop on the inside. I've always liked this idea but never tried it.