SNES and NES are the two systems I like to collect for.
NES games are usually cheap, barring a handful of later games and some oddball things.
SNES is different. I bought a retrobit wireless controller and exactly one game this weekend. Super Off Road and it was $25ish. I just buy a game every few months or so, slowly expanding my collection. The secret has been out for a while about SNES collecting, but it's not too bad if you actually *PLAY* that expensive RPG you purchase. You can get a few dozen hours of enjoyment and either hold on to it or sell it for more than you paid.
I literally collect for the 2 most poplar systems there are to collect for.