I would say they're worth next to nothing since they aren't proprietary in any way. They're the same adapters that could be purchased at any RadioShack or TV store in the 70's, 80's and 90's just with a SEGA label.
The only SEGA-branded thing I'd say is worth a little more as far as power goes is that SEGA power strip they produced with extra space between the outlets since their AC adaptors were so freaking massive :P
Probably much rarer than we think today. I'd think many have been long discarded.
I doubt much interest except for collectors. Or those who to a baffling extent don't care about video quality, Genesis & SMS are infamous for their poor composite/RF.
Lum fan.