This is going to sound harsh, but you bought an 8 year old BMW. Just because it did not cost 60k as it would if it were a new 3 series convertible does not mean that the parts and labor to keep it on the road is cheaper. The price they are giving you for the brake job is not bad at all. Be thankful that all it needs is brakes.
Start reading the wiki and educate yourself for what preventive maintenance you should be doing as well as other common repairs for that car. Then put the top down, go for a drive and have a some fun. I did just that this earlier this morning.