@(you need an account to see links) $1, most likely, maybe $1.50.
They're only 100 NC on site, so usually offering 1 cap (which is worth 200 NC) is a more than fair offer.
I wouldn't go any lower than $1, though, because they aren't always buyable in the NC mall and sometimes people seek them desperately after everyone else has bought up the current stock. xD
Also because of boxes. 'Nuff said.