Many NGCD newbies start with the newer titles at first and work their way down to the early games like FF, AoF and WH, and then they realize that it's much easier to get stuff like AoF3 or the newer KoFs than the classics because, like NBM and others already mentioned, the production # weren't as high as, for ex. for games that came out in '96 and '97, a period where SNK seriously tried to shift ppl's focus from carts to CDs, and the demand is also there, mainly caused by collectors that want to complete a series.
However, this isn't the case with all first parts, for ex. SS1 is all over the place, it got released in higher numbers because of its arcade success.
Then there are these "off-broadway" games, you already mentioned Robo Army, Burning Fight, etc. - same ol' song, low initial release numbers, ppl didn't care for them at first, now they are quite sought after but hard to find complete - Sengoku with spine card? Good Luck. Sealed?? Uh oh.
More hard-to-find games are Overtop and Neo Driftout, for the last two years I didn't get a single offer for them... damn.
Magical Drop 2 is easier to find now, but for some strange reason (...), it's increasingly difficult to find Zintrick CDs...
