Some people prefer 20's because it's easier to shoot from the phone with 20's.
I have well over 150 of them, and the most I ever paid was $25ea for a few NIB 70 dated AF contract mags.
The rest I have paid between 10-15 for, and even at that price I have been disappointed because some sellers are not honest about condition.
With the 20's you have to make sure the front strap isn't rusted (it's spring steel), and that the rivets are not popping out.
Many mags were not taken care of, and suffer from rust and dissimilar metal corrosion from repeated weather cycles.
While mags are designed to be a consumable item, the military doesn't treat them that way, nor do owners.
When selling used 20's, sellers should be honest about the condition. I've seen several ads where the seller is asking $20+ for mags with corrosion issues. That's just blatant dishonesty IMO, and part of the reason I haven't bought any mags in a long time.