$750 for a half hour job and a 25-30 buck cable!!!! Did the dealer smile as he took your money? He should have had a huge one.
Oh and cables - our current 5er is an early build '14 and to date all cables are fine though do need adjusting too often. Our last and SOB 5er was an '06 and the bedroom slide was a BAL cable and still going strong in 2014 when traded.
Watch the BAL ACCUSLIDE cable repair video on You Tube. It sounds like you have a major misalignment problem