Quoting chrisfullwood:
Check your Compression, that could give you an idea, of whether its motor, or turbo, also look at the oil itself, is it sooty?
Could be a variety of things.
Work piston rings, leaving valve seals, Leaking exhaust side seal in the turbo charger.
Price, who knows... depends on what it is, and what combination of other parts you have to replace.
Just curious, what would cause "sooty" or very dark, dirty oil if you are losing oil?