Weird one this. Sega is effectively encouraging buyers to buy all four variants to cover the full catalogue of available games. Furthermore, AFAIK, buying all four game gears is the only way to get the lens to magnify the image from the minuscule 1.15" screen.
Buying all four game gears will set you back around £160 (+ exchange rate fees, shipping, tax etc.). So maybe what - £200-ish give or take, delivered to the UK?
I have the Megadrive, SNES and NES minis and this would make a nice novelty gift... but not at that price!