What XM radio are you getting? Is it something you're buying from Nissan? If you are getting one similar to mine... then NO! it is NOT worth $400 for a dealer to install. I installed mine myself. Just need to know some basic eletrical stuff and mount it somewhere. Of course I did it on my '03 and can't say how easy or hard it is on an '05. But if you're just getting an add-on one that's going to just plug into to your antenna... then do it yourself.

**edit...
Is this the one you're looking at?
Xterra Parts - XM Satellite Radio Tuner Base Kit