I think I've seen some people quoted at $5-600... IMO anything up to about $800 is reasonable for the whole ball of wax. Any more than that is getting into serious ripoff territory.
Thanks to the knowledge and friends I've picked up on several Nissan boards and clubs (including, obviously, this one) over the past few years I did mine myself last year for a couple hundred bucks
ETA: The job will be pretty much identical on any '00-'04 Xterra or Frontier. '05+ Xterras do not have a timing belt, they have a chain - which you don't replace (at least not at 100k miles!)
ETA ETA: WTF is with the extra hour for the tensioner? That's like 15 extra seconds to plop on a new one while everything is apart
Based on that alone, I'd look elsewhere - they either don't know what they're doing or they're in the back room rolling around on the floor laughing saying "oh - oh - and get this... I'm gonna get another $100 out of him..... for REPlACING THE TENSIONER!!!!
What a TOOL!"