I'm into my Road bike conversion to CX build for about $50 so far. (Including bike) See link below.
I've removed the 53 inch chainring since the pictures and have a line one some bar end shifters for $15. The only future investment might be a CX fork with Canti brakes. I've seen some used ones on eBay go for +/-$30
http://www.bikeforums.net/showthread.php?t=221616