The derailluer repair cost could vary quite a bit. For me I'm lucky enough to have a lbs with a good selection of used parts at low prices, I could pick up a good used rear mech for $5 or so. Ask at your local shop if they have any used parts to dig through. If you get new a good quality piece will run around $25. Next can you install and adjust the new derailluer yourself? If not it's not hard to do and is a good skill to have. The Park Tool website should have a good photo tutorial on the subject. If you have a mechanic do it should be around $5-10.