2002 BMW X5 using oil

hi, my car had thick white/blue smoke coming from the exhaust it seemed to get better then came back again then oil light came on and the oil was up the back of the car ive had the turbo replaced and the oil sepatator valve replaced and its still using oil a 45mile test drive used if i remeber rightly the garage said 3L of oil, they now stripping the engine out the car to bits to find the problem what could the problem be, im guessing its going to be very costly ive been told its repairable but not sure if it be cheaper fixing the problem or getting a new part of an engine any ideas?

thanks