Answer the question!
Doesn't seem like anybody's answered Greg220's question, which is, I think, "when do I know the legal and financial terms of what I have to pay for my car?"
Well, Greg220, if you have negotiated a price and have that down on paper on some agreement the dealer printed, look at the terms of that.
If that document doesn't exist or you don't have any other actual sales contract with the dealer, you just have to rely on whatever verbal understanding you reached with the salesperson. Whom you could ask again, now.
In my case my dealer (Passport) sent the formal sales contract, etc., alarmingly close to the day I was to leave for Munich--about ten days before, I think. That was to suggest I pay them before I pick up the car.
If you're worried about the price going up why not pay for it now?
Incidentally, it might not be inevitable that BMW raises prices in the fall. It is really quite amazing to me that the E90 is priced about where the E46 was, and has been for months if not more, while the $/Euro ratio has changed by thirty per cent. Even while BMW was enhancing the equipment (e.g., 6 spd standard on 05 325i).
Ah, what's a few per cent anyway? The fun value is worth it.