I have a friend whose dad passed away in 2000. He came to me the other day trying to sell a bunch of his ammo and stuff.
Bought it all for one price, regardless of I needed it or not.
He had a box with some mac 10 30 round magazines in 45, I have never owned a mac but I know he did. These had a gun show invoice on them showing they were bought in 1991, they appear to be parkerized and have a foil wrapper one them. All appear unused.
What is a fair price per piece?
I see prices as low as 30 to 40 a mag and as high as 100

Did you snap a pic, and do you know if the magazines or wrappers are marked? It would depend under which manufacturer they were built and marketed. Original Military Arms Corporation magazines will bring a premium, with the RPD and SWD bringing less.

Either way, you'd have to find the right buyer, someone who is interested in M10 history, as it's just a modified grease gun magazine, and several companies make release levers that function with un-modified grease gun magazines as well - so shooter mags are cheap and easy to find.

MAC 10's were made to use M3 grease gun magazines. I believe they may have made some of their own, but it sounds like you have M3 mags. Not bad property.