Thanks for the input. And copy cart! Wow, I've never heard of that until just now. That's so cool. I can't believe something like that exists. And yes, I'll stick with NTSC games as there's already more of them than I can ever collect.
Updated list...
BEST:
R6: The Official Frogger, Tapper, Road Runner
R7: Atari Video Cube, Demolition Herby, Up 'n' Down, Shuttle Orbiter
R8: Crazy Climber, Boing!, Avalon Hill, Death Trap
R9: Rescue Terra I, Quadrun, Q*berts Qubes, Bumper Bash, Cakewalk
R10: Music Machine, Mangia, Asterix
CHEAPEST:
R6: Pengo, Frogger II, Tapper
R7: Demolition Herby
R8: Rubik's Cube
R9: Quadrun
R10: Asterix*, Copy Cart
*I just noticed Asterix has a PAL version that's an R5. I'll leave it on the list since I'm looking at NTSC games. If any game has NTSC label variations of different rarities, I would only consider the least rare option for these lists. I'm not sure I'd pay a premium for the exact same game just because the label is different.