It's one of the most joyful things collecting retro video games, when you're able to get an item way below it's actual value/price.
I guess one of the most cost-efficient buys I ever made was buying two GB color coming with about 18 Games from a mother selling of her kids' old stuff not knowing what it was worth. I paid 30,- euro for everything, and those games included PokéMon Silver. Nice catch!
Another good buy was a PAL region SEGA Saturn (2nd. edition controller included) for about 70,- euro ... seller said it wasn't working properly, but he just didn't know that you can revive the internal memory by simply changing a litte battery in the back of the console ^^
Properly the cheapest - not even a buy - was that Wii console a friend of mine found between garbage that someone left at the sidewalk ^^ he gifted it to me and it ended up working flawlessly until this day, heehee ... only the foot was missing, but lookahere: exactly one year after that incident, he found the foot between garbage in front of the same house where he found that console! Life writes best comedy - I ended up owning a complete, working Nintendo Wii without paying anything.
Guess I'm quite good at profiting from the lack of knowledge of others, but hey, they got the amount of money they wanted