Not likely a lot of help - I bought two new ones from a CGN Sponsor - June 2016 - I paid $184 mailed to me in Manitoba - many years ago - might be totally different shipping / mailing costs today - but might give you a price reference to start with?? I have not looked for them since I got those two (10 rounds each - 7.62 NATO, for an AIA rifle), so no clue what they sell for these days. In May 2016, I bought a used 3 shot magazine in 308 Win for Rem 788 for $58 from CGN, mailed to me - and another used one in October 2016 for $65 mailed to me. More recently, in November 2022, I bought a used Parker Hale P/N 98/224 (308 Win, 5 shot) from CGN for $85 mailed to me.
If I am going to have a detachable magazine rifle, I chose to have spare magazines - over the years I have had them fall out of my rifle and lost (while hunting in bush), can't find the "one" when I am ready to go, bent some (sat on them) so they do not work, etc. - to me having spare mags is just a cost of having that form of magazine for the rifle.