You will be luky to get 10% on anything other than a FIAT tbh.
BMW and Merc. will try to give you nothing off.
Budget maybe 6% for a straight cash deal. Depends on the maker, car, it's place in it's life cycle etc.
If I were you I'd hold off until March, Dealers often give very attractive deals at the end of the 1st quarter when every sale matters to their rebates.