A fair price for 12 ga. hulls is free, if you shoot at a trap or skeet club. If you do buy them I wouldn't pay more than 4 or 5 cents. There are a lot of junk hulls that I would reload once but not pay for but the ones I would pay for and reload several times would be Any Remington (not UMC) older Win AA compression formed (not the newer AA 2 piece hulls, or the Universal)
compression formed AA 5 cents
Rem STS, RXP, Peters Blue Magic 5 cents
Rem Gun clubs 3-4 cents
AT Most!
Try this for costing,
http://www.trapshooters.com/rlcalcadv.htm