2001 chevy tracker oxygen sensor question

Does anyone know how much an oxygen sensor costs? A friend of mine took her Tracker to her trusted mechanic and he told her it was going to cost over $200. That doesnt sound right at all to me. I think there is a possibility that there is more than one because he said something about it being the upstream one. I looked on ebay and there was two. One had four wires and one had two wires and they were both under $26. If anyone has replaced one please let me know what your experince involved.
Thanks in advance.

A mechanic told me to be sure the one you replace is pre-wired with the same configuration and plug as the old one and brand does not matter, other posters swear by OEMs, they could just be working for the manufacturer or dealership, though. I've also read that 9 times out of 10 it isn't the o2 sensor that's bad.

I just replaced both o2 sensors on my tracker. First I replaced the upstream with a universal, trying to save myself money, and the code still showed up. After talking to a trusted mechanic, he said he hasn't had real good luck with the universal sensor. So instead of saving $100 I ate $70 for a sensor that was usless. I then bought the oem replacement. Then the engine light came on again. it was the downstream. I replaced that one and everything is fine now. I asked my trusted mechanic friend and he said sometimes only one will go, but more commonly if one goes the other isn't too far behind. The upstream sensor was a little cheaper than the downstream, but yes they both cost near $200 each. and if you have to replace your starter, be prepared to spend a pretty penny on that one too..