I don't know where people are coming up with these numbers. The sport tires will be good for about 20K miles if you drive normally around town with occasional spirited driving. If you're auto-crossing the car often, then yeah, don't expect more than 10K miles. But then, you wouldn't expect such a thing if you autocross.
Also, I don't think the standard all-season run-flats are $400. Probably closer to $300. This is easily checked at TireRack, links provided in this forum. That would be better than just randomly guessing numbers and misinforming people.
E90 330i delivered 6/29/2005, ZSP/ZPP/ZCW/Step, Arctic/Gray/Walnut