Standard used USGI 30rd aluminum body magazines for $4.50 each is a good deal as long as the feed lips are not thinned, cracked, or there is any body damage.
I was buying very good condition used mags for $5 ea in quantities of 5 or more about 6-12 months ago. And thought those were very good deals.
But if the mags in question have any problems, or are heavily used then I am not sure any price would be a good deal.
FWIW - NIW USGI aluminum mags can be had for around $7.50-$8.50 in todays market. Some folks would probably strongly argue that at these kinds of prices why not just get new ones. I understand that line of thought. Its just that I like saving money where I can. So if I can inspect the mags, I have no problems buying used ones.