Absolutely they should stick to their advertised price, at a minimum! If not, find another dealer.
FWIW, I got 5K off MSRP for my soon-to-be-delivered 2011- so I would think you can do at least double that for a car that is going to be two model years behind before long.
2011 E89 Z4 sDrive35is Space Grey Metallic/Coral Red/Aluminum Carbon trim; Premium Package, Cold Weather, Premium Sound, NAV, PDC, Anti-theft
2008 E60 528i
2005 E85 Z4 Roadster 2.5i (Retired)
BMWCCA Member #434186