I would say if your chain is skipping during riding you should replace both. Those teeth are probably more worn than you think. If you replaced it before skipping you should be able to replace just the chain. That said I would bet that if you just replaced the chain you would have no problems but would run into problems sooner (say 600 mi this time).

Also, if you are trying to save money learn how to replace a cassette yourself. In my opinion it is one of the cheapest and easiest repairs. Just but the tool for 6 bucks (your LBS is probably charging you at least 15 or gasp for a "tune up"). It is a simple thing to do just for lack of a better term, unscrew the old one, slap on the new one and done. The best part is no adjustments to fiddle with.