I've been to the commissary on JBER, but not Eilson/Richardson. The prices are higher than most of the lower 48, but might not be too much higher than what you pay in N VA. When you are assigned to JBER, you get a pretty hefty COLA, which is usually indicative of high costs - BUT - in Alaska, those high costs are more dependent on utilities and not groceries. In fact, while we were stationed there, every active duty member had to submit utility bills for X number of months, which resulted in the COLA being raised.
The meat counter at JBER was always pretty good, but the ribeyes were better marbled and more tender at Sam's. Never went to Cosco, so can't speak for that. While you are in AK, you should *REALLY* consider getting groceries at Fred Meyer. They have awesome sales, and it is super easy to accrue gas points. Most FM stores have gas and diesel in the parking lots. We usually averaged $1.50 off per gallon by shopping at FM.
Also, no taxes in Anchorage, so there is no benefit to the tax-free commissary.
One thing you can get at the commissary cheaper than off-base is King Crab. The commissary carries 25lb boxes for CHEAP!
2012 Dutchmen Voltage Epic 3795
2010 Dodge 3500 DRW MC
2009 GMC 2500 DA CC
2012 Smart-for-two (sometimes hitches a ride in the Voltage!)
2005 Sundowner Sunlite 777