Sorry to hear it dude that sucks.
I would check NADA, Kelly, Edmunds etc. as reference points. You can also look at the used car advertisments on Cars.com/Autotrader/BMWUSA. I would also check the BMW CCA classifieds as well. Then depending on the condition of the car, how well you kept records, etc. you should have enough information to come up with a fair value. Good luck.