I think converting to a triple would require a new front derailleur and shifter....that may be where some of the expense is (and that may make a $500 pretty cheap, actually).
Still, can't you put a road compact crankset on it (a 50/34)? I don't think that would require anything other than the cranks. You could get a compact crankset - for instance the
FSA Gossamer with a MegaExo BB for pretty cheaply I would think...
ETA:
http://www.performancebike.com/shop/...tegory_ID=5122