are you saying they want $400 total (100 more than just buying the alternator?) or price of alternator plus $400 labor?
If you have experience changing the accessory belts, changing the alternator is not that much more difficult.
You have to take off the wheel, and the plastic covers, then you can reach the A/C belt. Loosen the tensioner, remove the belt, now you can go back up to the front drivers side of the front bank and start to remove hoses and aircondition lines (don't have to remove, just unbolt and move a little to get out of your way)
Now you can unbolt and wiggle out the alternator...
Reverse to put back together
and yes, I'm oversimplifying this....
I think you can get instructions on removing and replacing the A/C belt here: http://www.stealth316.com
removing the belt is 75% of the job.