Seriously, any time you start collecting for a given system – buy the rares first. You’ll get less games for your money, but in the long run you’ll save a mint. Crappy NES games keep falling in value, but the rares keep on climbing.
Case in point, Bonk’s Adventure. I remember when a loose copy first broke 100, people though it was insane. Now it can fetch a decent price on its own -always. A complete version can fetch considerably more.