don't think of the up front cost on snow tires/wheels, think of just the wheel cost. Eventually everyone has to replace tires, so having "extra" winter tires will save the life of the summer tires, Say you would have got 25,000 miles on your summers, now those summer tires will last 35,000 miles because you put 10,000 miles on your snow tires over a 2 year period... Your only real long term cost is getting extra wheels, and if you go to tirerack.com you can get a decent looking set for $600-$800, if you can afford a new BMW 3 series, you can afford that and the extra safety snow tires will give you in the winter, plus you don't risk damaging your BMW OEM wheels while driving in the winter.
doesn't matter if you are getting a 328 or 335, both are "performance sport sedans" and deserve summer tires, otherwise go buy a Volvo S60
