I'm not a full-on collector (I only buy what I want to actually play), but I have built up a nice collection over the years, but my main rule is never go over €200,- fo any one game. This is only reserved for games I really want and have no realistic other way of obtaining (I don't do emulation).
With that in mine I ended up buying a complete in box and close to mint Earthbound about 12 years ago for what I then thought was the ridiculously high price of €200,- It ended up being both a great investment as well as one of my all-time favorite games, that I still proudly own. Looking at pricecharting, it's worth around €800,- and I rarely see one in great condition like mine is.
Outside of Earthbound: Panzer Dragoon Saga (PAL, not quite as highly valued as the NTSC one), Chrono Trigger mint, FFIII mint, Demon's Crest, Rondo of Blood, Cubivore, and lots more €100-ish games like Phantasy Star IV, Winds of Thunder, Fire Emblem Radiant Dawn etc.