If you have access to Japan, there is a ton of really cheap games in the local Book Off, TRADER, and other small game shops.
I bought a top of SFC games at 100-500 yen each ranging from Yoshi's Island for 250 yen in box to 25 yen copies of Street Fighter II Turbo to 600 yen SMW. Most are reasonable and a lot of times you will find the SFC version to be cheaper than the US version. Of course RPGs are out of the question. I have few dozen games but I will be getting more when I head back to Tokyo in a few weeks.
You will see the off color ones are way cheaper, I got one copy of Zelda Link to the past for 100yen with an slightly yellow back.
I'm only looking in Osaka for rarer games now. I live in Fukui prefecture, which is inaka, and I've bought every game of interest I can find at book offs, comp offs, hard offs, the local recycle stores, and a chain of awesome used game and anime merchandise stores. The latter are great for the range of prices, and I picked up all the Dragon Quest and Final Fantasy games for much less than their common prices by stumbling across copies that were hiding behind others and had much lower price tags (most for 100-200 yen). I found 400 yen Terranigma and Illusion of Gaia, 50 yen Magic Sword, Star Fox, Ogre Battle, and Final Fight, 100 Yen Secret of Mana, 100 yen FF6, and 500 yen near mint in box Dragon Quest 3 SFC remake.
The only problem I have now is my tiny school owned teacher's housing apartment has virtually no storage space, and I'm running out of room for all the games I've picked up here. I've bought about 70 SFC games, along with a mountain of GB, GBA, PSX, N64, PSP, DS, 3DS, NGP,WonderSwan, and Famicom games over the my time in Japan.
That said, the local Book Offs seem to have gotten the biggest restock of the year recently, and I snagged a few games I had been looking for for ages, incliding the Wonderswan Klonoa.