replacement NIssan OEM cat is around $500-$600. I replaced mine with an Eastern Catalytic Direct fit unit for about $200. The fit wasn't as "direct" as they'd have you believe, but it did fit with a little muscle-ing. Overall it's a smaller unit, so getting it in the spot is easy, but the flanges weren't welded on at the right angle.
The Cat itself did not fail, what did fail was the exhaust flange, which rusted through at an alarmingly fast pace, and left much of the exhaust dangling.
Dealerships are what they are; high cost labor, high cost parts, piss poor diagnostics, but generally quality finished repairs with factory parts.
I find it's pretty easy to negotiate part prices with them though. I recently had the factory tow package installed on my Quest; they wanted to charge me $650 for the receiver and $275 for the harness... I got it at $400 for the receiver and $125 for the harness. Still more than a U-haul unit, but looks very nice.