I would say it's what ever the seller wants for a certain magazine. As far a over all quality they are all pretty much the same. There were many dozens of magazine makers for the military during WW2. But some people just have to have what they may think is a rare one or one that was made for the maker of their carbine. So they wind up paying top dollar for one that is no better then the others. Kind of like a women who just has to have a matching set of shoes for her dress. The average price is mostly between $15 and $20 per 15 round magazine. The so called rare ones can go for upwards of $100 and in reality they are not rare as they made tens of thousands of them.
That's been my experience. The prices don't seem to have a rhyme or reason. just priced by what the seller thinks. I've bought them 5-30 rounders for $25. Then some people want $50 for 1- 30 rounder, saying it's rare! There probably are some worth more than others , but I haven't done any research on it. All I know is I have a butt load ,with many various markings and they were all cheap. Somebody probably actually knows.
I bought 50 Taiwanese military grade magazines new in the wrap and grease last year, I think I paid bout $12 each. I bought a original condition m1 carbine couple months ago and it came with 9 us military surface rusted magazines, 7 of them thirty rounds and two of them fifteen rounders, two of the thirties were sort of silver soldered together, and I have a group of other shooter mags newer made in korea, and various lightly rusted and used fifteen rounds I picked up over the years, probably going on a hundred mags, none of them particularly valuable, the fifteen rounders are the small number, doubt I own fifteen total fifteen rounders. Oh, I have three of them ten rounders that came with some big 5 m1 carbines recently imported, they are well made at least.
Maybe I am magazine hoarding, probably someday will sell off some Taiwanese mags if there is any money in it of course.