Kalkhoff is a German bike company, and that's a folding bike model of theirs. I'd guess from the 1970s, based on the cottered crank, but it's a pretty wild guess. They're not common in my little corner of the world, but rare doesn't necessarily mean valuable. Unless you're talking about a Brompton, they don't command much money here. (That said, I wouldn't mind having one if I stumbled across a deal.)

There's a folding bike forum here on Bikeforums and the guys in there would probably be a whole lot more helpful than me:
Folding Bikes