Conversion carts are nice especially when it is a game that is released Arcade MVS only. I only got one which is Shocktroopers. My Neo Geo collection is balanced between expensive and cheaper titles. The games I had to pay roughly $265-$315 when they were US releases over a 4 year span is; KOF 99, 2000, 2001, 2002, Metal Slug 3 and 4, Matrimelee, Mark of The Wolves, and Rage of the Dragons. I sold all the games that were made by Playmore since I just felt the quality wasn't there compared to the original SNK games. That leaves me Metal Slug 3, KOF 99 and 2000, and then Mark of The Wolves. KOF 2001 is by far the biggest embarassment of the KOF series. The worst backgrounds, music, and character art ever. I gave up on Playmore Neo Geo titles after Matrimeele and Metal Slug 4.
This money helped me buy other games around $200 like Last Blade 2, Art of Fighting 3, Samurai Shodown 4 and other cheaper original SNK made titles. Still had money left over thanks to some games selling higher than I bought them for, but more of them had rapid price drops due to just being non-desirable titles. I am glad I sold a lot of them early, which would actually make me money if I rebought them now. Neo Geo stock market: Sell high, then buy low. US Mark of The Wolves goes for $650 atleast.
I am not really rich or anything, I just bought 2 new games a year. Most Neo Geo titles nowdays you can get for like $20 to $50 easy. It only gets expensive at $80 to $200 range if you buy games that were released from 1995 and up usually. Here is a price list, you would be suprised how much less you can get some of the games listed. It is fairly accurate.
http://www.neo-geo.com/guides/aesguide.html