everything is expensive on a bmw. either from a dealer or a good independent shop. Call around and ask how much a brake job, muffler replacement, etc. is. Or look on line for replacement parts to see how much they cost. Not trying to discourage you, just be realistic.
I've seen several times that you need to be ready to spend $2k to 2.5k per year on repairs/maintenance if you don't have a warranty, depending on the age/condition of your car and your driving habits.
Just my observations from owning a used 320is for 13 years, other 5 owners may have more exact $ values.
2004 E60 530is, Delivered 12/12/03, TiSilver/LGrey, PP, SP,PS, PDC, ACC, FRS, Step, Xenons, clear bra, body colored rear inserts. (Retired, 1983 henna red 320is)
2004 Corvette Z06 Commemorative Edition