"free kittens" deal

Refers to an offer/proposal/situation where the items or circumstances appear to be favorable or legitimate as they are initially presented, but then turn out to be a "bottomless pit" of required labor, responsibility, and/or financial resources.

A brand new ink-jet printer for only $29.95 may seem like a bargain, but it's actually a "free kittens" deal, because replacing the ink cartridges will cost more than the printer.