OEM/dealership was ridiculous when I needed to replace my 98 DX rear o2 sensor...they wanted nearly $300.
If you're a cheap bastard like me, you can get a generic 4 wire o2 sensor off Ebay for under $40. You'll have to transfer the old sensor's plug to the new sensor, but that's just soldering four wires.
I have a bunch of fairly new O2 sensors off the cars we stripped to make race cars. Send me a pic of the one you need and I'll check to see if I have it....