You might check Liquidationtime.com. They have auctions for BlueTooth headsets all the time...can't say for sure what they end up selling at, but it's a good place to start. Also, SurplusComputers.com has a decent one for 25.99 (I'm using it, and I like it) Might be a bit over your range, but there it is.
Good luck...let me know if you find some other good places.