chevy chase

well got my rotors on, and my mechanic scaned my ride while he was at it(check engine light was on)found a, o2 sensor code, which makes sense cause i only get about 275on the tank(highway miles). any way, is this an easy do it yourself job, or a mechanic only job?i chekcked advance autoparts and they said 60$ per o2 sensor, does that sound right?

where are the 02 sensors located???(slk230 1998)
thnx gents