My vote is on the light end of the scale -- a couple spare cables, at least one set of brake pads per species used, two tubes per bike, one spare tire per size used, a multi-tool per bike (in case the last person has to fix something). For everything else there's a bike shop within a day's drive.
One thing I haven't see mentioned is a 6-outlet power strip, so everybody can plug into the one available outlet in the motel room and charge up all the phones, GPSs, e-readers, and all the other things that Absolutely Must Be Taken.