Gilberts charges $25.00 for the FFL transaction, and nothing for additional guns on the same order.
Dawsons Small Arms in Woodbridge charges 50 bucks and is annoying. Stay away from them.
Blue Ridge Arsenal in Chantilly charges 50 bucks.
Loudon Guns is EXPENSIVE so keep your wallet in check. I have no idea what they charge though.
If you want a kitchen table FFL, go through Joe Elarde in Leesburg. He charges 25 bucks and his prices are about 150 dollars below everyone elses. PM me if you like for his E-mail address.
VA Arms, haven't had a problem with them. Good staff, good inventory. Hours are kinda crappy with them closing at 5PM and not being open on Sundays.
Guns and Ammo Warehouse down the road from VA arms has a damn good inventory, but charges 40 bucks for a transfer and their other prices are high. However, they do have a hell of an inventory and more often than not you can find what you are looking for.
Looks like Gilberts or VA Arms is a good choice. However, it depends on where in Northern VA you are. If you are in the Alexandria / Arlington area, Gilberts is your best bet. If you are further south in Woodbridge / Manassas area, go with VA Arms. The two are far enough apart to justify not driving that far to save 5 bucks.
Good luck!