I just had my transfer case resealed as it was leaking. The dealership charged me ~$750 to get to it, do the reseal, etc. There is a way to get to it by removing the front frame cross member from what I'm told.
If you find a used one, or find a kit to rebuild yours, then you should be able to get away from the mechanics for ~$2500. Thinking the price you quoted was probably for a new Nissan part plus the labor to pull the entire motor/CVT and do the replacement.
After 142K miles, my MO is history. Now sporting a loaded silver 2012 Ford Edge Limited with 20" wheels.