Title says it all...what would be cheapest? Region-modded white PCE w/ AV booster? Or composite mod? Core Grafx? I'm looking to get a cheap setup for m'lady so that she can play Bonk and Granola Snake.
Core Grafx I or II.
Not much need to region mod it if cost is your main motivator since at this point most TG-16 games are much more expensive than PCE. With a flash card they are even cheaper.
Just get her an LT so she can play on the toilet.
What Zeta said. By tapping the expansion port for composite (essentially free mod), a TG-16 or white PCE would be good options too if found cheap.
TG-16 regularly hits $100 working, a plain old white PCE is usually found half that price. PCE does not have composite port so one will need to use rear EXT port or mod and install a port inside. Removing stock RF and installing 8 pin mini DIN wired the same way as Sega Genesis and Megadrive works the best.
PCE has one benefit: works with almost all controllers. TG-16 only works with TG-16 without adapter.