Something not discussed, but essentially free is the removal of sound deadening material which is typically bonded to various spots of the chassis in areas where sound penetrates the passenger compartment. I still have an E46 M3 and pulled approximately 30lbs of this stuff from trunk and rear seating area. I'm certain that there's more, but the car is still a DD.
We're 5 weeks away from picking up my wife's E90 M3, and I'm hoping that this will be a relatively easy mod to shed some weight.
I have some tips for easy removal of this stuff if anyone is interested. The only costs involved would include a heat lamp, a few PLASTIC paint scrapers, and a can of carb/choke cleaner. Oh, time to remove the trunk liner and rear seats and carpet can't be discounted either.