Is the upstream O2 sensor a wideband or narrowband?
 

Just curious. I thought newer cars use wideband sensors, but someone at my shop (who probably doesn't know a hell of a lot about the RX-8 specifically) said it's a narrowband sensor. I'd like to know which it is, so I can anticipate the cost of eventually replacing it.

Most cars have both as narrow band.

The RX-8 has a wideband in the front, a narrow band in the rear.

I thought that might be the case. So, how many hundreds of dollars does a replacement upstream sensor cost?

Depends on where you buy it :)

And no, I'm not going to go price shopping for you. ;)