Which month/year and engine do you have?
Cheapest one is:
The ventilated 4-piston brakes from the Germany only version for the 750i/iL built from 01/1990. Front 324 x 30 mm. All other E32 have in front 302 x 28 mm.
Interesting are also the 2-piston Brembo brakes from the E31 840i from 03/1994. They also have 324 x 30 mm but brake as well as the old E32 German version 4-piston brakes.
But to tell the thruth. Nothing is cheap. The German 750 version is rare to get used, as all in Germany want to modify to these.
Alternative might be new Brembo.
PS: I hope I did not mix up the numbers
you can check the brakes of the E31/32 here
http://www.realoem.com/bmw/select.do...938848d4d39ea1
Also check for the E34 M5 brakes during the last production years.