The asking prices for dummy cameras varies, and does depend on the model (some are realtively common). But many remain unsold on dealers' shelves for months, or longer. For example: a M3 dummy may cost £295 (see MW Classic website) but normal dealer prices for real M3 cameras, in good condition, would normally be £100 or so more.
For the 'flex look at the Leica Shop and enter 'dummy' in the search box: most of those display cameras have been there for a year or more. It's a buyer's market; hence, as EBay is where buyers (ie collectors) congregate there may be some past auctions you could check.