We have a 2007 Nissan Versa with around 40,000 miles.
At 18,000 miles we were told we needed a new Oxygen sensor, which was covered under warranty, now at 40,000 miles we have the exact same problem, this time it is not covered and will cost around $300, and has a 1 year or 18,000 mile warranty. seems to me it is going to be an expensive fix every 20,000 miles or so to replace Oxygen sensor.
Any one else having this problem

$300!
Why so much?

We took the car into Suburban of Farmington Hills as SES light on, there's $165 to start with just for them to look at it, then $112.71 for the air sensor and $165 labor, my wife likes the car, but I can't say I would ever consider a Nissan again based on cost of ownership there are multiple recommended services and none of them are cheap, and that is before the SES light decides to kick in on a regular basis.

you probably have a lemon; no way would an oxygen sensor go that bad early in it's lifetime...