The Neo Geo CD had 7 megs of RAM, and the Neo Geo cart games just kept on getting bigger. The last CD games released would often load midway through levels or several times per fight for fighting games. Then from there many Neo Geo games never got a CD release because it just wasn't feasible.

As for the price of Neo Geo games, just for some fun here is one that sold last week for over $5000: http://www.ebay.com/itm/Blazing-Star-Neo-Geo-AES-JP-import-1...

Many Neo Geo games are now in four figure territory.