They came later in the Commodore 64s life when Commodore was looking for something to compete with the NES, something simpler and cheaper than a full-fledged Commodore 64. So they are really from the second generation of consoles, not the first which included Atari and Intellivision. They are basically a Commodore 64 stripped down, only able to play cartridges.
That said, the are actually much rarer than real C64s and therefore go for higher prices than a regular C64 on eBay. I don't know exactly how much more. They weren't really rare, but they aren't common either.
