When you take it to the stealer, they replace the entire water pump housing, which around here costs ~$140 in parts alone. The impeller itself only costs about $50 for the cheap plastic one, but I'd recommend getting the one with the metal impeller from NAPA. I think it only costs a bit more (maybe ~$80), and it won't break. Replacing only the impeller part takes about 20 minutes (An hour tops if you're not mechanically inclined). Replacing the entire housing does take a bit more work.
Do a search to find the part # and how to replace it - it's been mentioned many times before.
