I think that Lagenator is right regarding the M1A mags. Springfield is charging some really obscene prices: $39.00 for new 10-rounders
and
$95.00 for USED 20-rounders (see
http://www.springfield-armory.com/shop/?page=shop/browse&category_id=2c839a27da7029919dacab8c9d92a170&ps_session=496162310343aeecbd0561a61f189996 if you don't believe me. Surely, someone out there can manufacture a high-quality 20-round mag for this fine rifle for under $20. Perhaps Springfield itself will do so, to pre-empt competitors (gee, ain't the free market a wonderful thing?).
Prices will fall considerably for most mags after an initial rush to purchase - probably 3-6 months at most. The cost of these things is on the order of $2-$3 each, at most - they are made of metal stampings, springs and plastic, for crying out loud!
Me, I'll be stocking up on all of my mags - I won't be vulnerable again to a ban, not from that perspective. I'm waiting, though, for prices to drop. Heck, the old and USED pre-ban mags may end up being real bargains! I'm also waiting for the 25-round 6.5 Grendel mags to be produced by several companies - I figure that the upper for the standard (or not-so-standard) AR-15 lower can be bought at any time, but the accessories may not be so available. See
http://www.65grendel.com/ for more on the 6.5 Grendel (no, I have no financial interest in the company - I'm just a consumer, like everyone else, but I
like this round.