inferno715 is correct.  At the last gun show I picked up a few used mags for $10 each.  Bodies and lips were in good shape.  One had a green follower, two did not.  Very little finish left on the mags.  The mags all function very well though.

Eagle Firearms is selling new LaBelle and Colt 30-rounders for $20 delivered (if you order 5 or more shipping is free).

Now, what about after the ban sunsets?  Well, look at the cost of LEO mags now.  A LEO friend of mine said their department pays ~$13.00 per mag.  I'm sure other departments pay less and some maybe pay a little more.  This is just what HIS department was paying for them.  So, what companies are tooled up to produce new USGI mags these days?  Will companies start cranking out a bunch of USGI-quality magazines for the civilian market right after the ban sunsets?  If so, will these truly be the same quality as USGI mags?  Frankly, I don't want a magazine that does not at least meet USGI standards.  I couldn't care less if USA Magazines starts cranking out mags.  I only want magazines made by the same companies on the same equipment and using the same specifications as USGI magazines.

Guess what I'm trying to say is I don't think mag prices are going to get a whole lot cheaper right after the ban.  Demand will be high.  New supplies I just don't see flooding the market anytime soon.  Plus, I really think we'll see another ban in the VERY near future.  

The only thing I might be missing here is if there exists some warehouse full of USGI magazines made after September 1994 that the military does not need/want.  If that's the case, I can see mag prices coming down, but still don't think they'll get below $15.00.

One other thought.  When the ban sunsets, it "MAY" be legal to own magazines stamped LEO/Military Use Only, but I'm not interested in any magazines with those markings.  I think it is asking for trouble.  Now, a date stamp is different.