Labor should be around $250/axle. That's based on a 3-hour job per axle at a shop rate of $75/hour and rounded-up for sales tax.
Parts kits ("Master" parts kits, including all bearings, races, ring bolts and shims) are about $125/axle.
The actual ring & pinion sets are about $150/axle.
So that's around $1100 total, give or take. Let's say $1200.
If you needed a different differential carrier, they are average $150/axle.
So then you'd be in the neighborhood of $1400-1500.
If you were installing limited slip or locking differentials, there's no additional labor time or cost other than the differentials themselves (EXCEPT electrically or pneumatically-actuated lockers like ARB/etc). You would not need an open differential carrier in this case, but tack on the price of the limited slips/lockers. Then you'd be in the $2500 range, depending on the locker.
If you like this shop, I would ask for an itemized quote.
For that much, they are probably subbing it out to someone else and making $1k for their trouble.