I searched all of the E90+ forums i know for a pinout of this "JB BMS USB Cable" and i couldn't find a thing. So i was stuck buying it for $35.
Step 1) Buy everything you need from a site like Mouser or Digikey
buy a TTL RS232 cable (5V) for $20
buy a DB9 Male (pins not sockets) for $0.43.
If you're like me you have these in your parts bins already
buy a clamshell for $0.65 (or buy a nicer metal one for $2)
Step 2) solder the black wire to pin 5 of the DB9 Male
Step 3) solder the red wire to pin 3 of the DB9 Male
Step 4) solder the yellow wire to pin 1 of the DB9 Male
Step 5) assemble clam-shell and you're done!
I know this only saves $10-$15, but the principle of the matter is that you shouldn't charge more for something by playing on the ignorance of your customers.