Marco the Neo-Geo market is very odd but utlimately its for very very serious collectors, for me I just wanted to play the games so I had an Arcade PCB converted to a console so I can play the Arcade versions of the Neo Geo carts on my TV The Arcade versions which are 110% the same are very readily availible and you can pick up Metal Slug etc for £30

These are the games I collected over the years