I don't have a dollar figure to give you, but the Duo Dock Plus is the only one fully compatible with the Duo 280c and 2300's (although other Duo's work in it just fine). Only real difference is the Plus lacks the FPU and cache that the other Duo Docks have.

You'd probably want to crack it open and see if there are any NuBus cards or a hard drive installed, that would help the price. You'd also want to be fairly sure the thing worked (do a google on "duo dock tick of death" for a better description of the common problem with these).