Usually good mags go for 20-25pds, depends on the mag cell some could be a bit more expensive.
The basic sato (5/155/45/0) goes for 20pd.
Other kind of mags without pbs(or with 1 pb - in case of being elenor) with the stats 5/195/0/0 or 5/0/0/195 usually come from 10-15pd, all here is a bit subjective, everyone prices their own stuffs the value they want.
Custom mags from mag makers might be a bit more expensive