Ask questions, meet dancers, and be part of the conversation.
what's your level?
If technique is what you want, for standard I would look at Ian Gillett who teaches out of Chevy Chase Ballroom (also next to a metro stop). For standard, I imagine both Lydia and Nonci would be good. If you want a male teacher, then Nonci.
For group classes, I would take the Chevy Chase Ballroom ones, taught by Ron Bennett. Can't imagine the instruction at Du Shor's classes would come anywhere close to those taught by Ron.
I'd say you should expect to pay at least $70/45 minutes for the real quality instructors.