They are pretty rare, but when their on Ebay you can get them between $10-20. with a game or two, the games themselves are rarely over a couple of $$.
It was supposed to compete with the gameboy, it has about the same graphical capabilites, but its just a cheap knock off that never caught on, but it still had a library of almost 50 games.
ALSO: Watara was not the only company that made Supervisions, there were a few different companies and a few different versions, but all in all, they are all Supervisions. I know the Watara had a tv connector, kinda like the SNES had Super Gameboy, to play Supervison on the TV.