A Sonic game for Master System recently sold for $981.33. When other copies of Sonic regularly sell for $30-40. The games are identical except for a small UPC sticker on the back of the box, as seen below:
Sonic the Hedgehog (Master System) is unique.
The Master System was much more successful in Europe than the USA. In the early 90's Sega released the Sega Genesis in the USA as the successor to the Sega Master System and very quickly cast aside any Master System support.
Sega published Sonic the Hedgehog in Europe because the Master System was still selling well there, but instead of producing the game for the USA they decided to import some of the European versions and put a UPC label on it so it could sell at retailers in the states.
The USA and the European Sonic games for Master System are literally the exact same game. The only difference is the small UPC sticker placed on the US version.
The game has become notorious with game collectors because the value is so different but the "rare" version can easily be faked with a UPC printer and labels.