The D3 isn't even expensive in the first place. I'd not have thought it would be a viable option to pirate the idea. Certainly, you could stick a cheap derailleur on an approximately similar frame, but it won't give the same service and pleasure as the original. Clones like the Merc are functionally and almost aesthetically the same as the Brompton. This thing isn't really in any way the equivalent of the bike that inspired it - maybe it was inspired by the curve, or maybe not. It's not that easy to tell.