I usually buy surplus mags at Nation's Gun Show and rebuild as needed. I did just order 10 gen2 Pmags from Proven Arms for $102 including shipping to a remote location. I'll take a road trip when they arrive.
Green top in va use to have sales on mags if purchased in boxes of ten but idk what they are doing right now since I have not been there since fall last year
I miss the Proven Arms retail store in Woodbridge, were the best prices around on most magazines IME.
Big Dogs has lots of magazines, but are retail or above on most of them nowadays. (and really high on ammo.) I think they realize how close they are to most of Southern MD and charge accordingly for whats in demand here but not available here.
DK Firearms 25 or so more minutes south of Big Dogs are cheaper, at least on some magazines. (last ones I got were CZ Scorpion 20 and 30 rounders and they were $10 and $13 less per magazine than Big Dogs)
PSA has them for $8 occasionally. I think you get free shipping if you order in quantity. For 100 or more it comes with a free bottle of liberal tears which is great lubricant for ARs.
I took a ride over to Leesburg,VA here from monkey county going across the point of rocks bridge and then south on va rt15 when i purchased about 15 30rd pmags. They were on sale at the time for $10 a pop so I made sure the trip was worthwhile..