Cost of O2 sensor welding
 

What is a reasonable price to pay to get a bung welded for a wideband O2 sensor?

Also - what is the optimal location for one?

Anywhere from $30-$60 (I paid $45) Also I would make sure you get a good quality Stainless Steel Bung. As for location, if Catless:
https://farm8.staticflickr.com/7365/...0c2f033c_b.jpg

If Catted right before the Cat:
https://farm6.staticflickr.com/5547/...33e5a6ec_b.jpg

You may want to mock up the Front Pipe in order to get the correct angle and location for the O2 bung.

^ this

I'd say $25-75 depending on if bung is included.

I marked with a sharpie where I wanted the bung on my overpipe, and brought it and the bung to an exhaust shop around the corner. They charged me $20, and had it done within 5 minutes of me walking in.