$15-$20? PM me here I'll buy them. $34.95 at MA Firearms School. $30-$35 is average price. Any reason you want 30 rounds? The market is glutted with Colt 20 rounders. Can be had for $20 every day. Might even get them for less if you buy in quantity.$15-20.
Maybe $25 tops if it were NOS with external date stamp.
Think about how many millions of them are out there. Don't rely on the NES classifieds only. Look around on other sites like ar15.com or m4c. It takes some patience to find them for less than $20, but they are out there.
One thing is clear.. they are not hard to find.