Retro and auction specialists Game Sniped have put together a list detailing the "Rarest and Most Expensive" SNES games as of May 2015. Don't let the title scare you; most of them can be had in some form for around the $US100 mark.
Not all though. While Earthbound is "affordable", prices skyrocket when you're after rarer games in a complete set. Zero the Kamikaze Squirrel, for example, is selling for around $US2000, while Incantation is going for thousands.
The list also has some advice for people shopping for games, and not collectibles; Syndicate is rare and expensive if you're after a US copy, but the PAL edition (with slightly cleaner box art) is cheap as chips.
You can check out the full thing here.