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The best place to tune a car is at a shop with a dyno. Cost depends on how long you are there, who tunes your car, and whether or not you will be using a wideband O2 sensor when you tune it.
What do you mean by "only for those between $7000 and $15000?"
If you're looking to buy an M3 and modify it w/in that price range you're probably going to want to look at 1995's. Check out Turner motorsports, they're a great dealer for BMW parts.