In a F series you can use a used MA61 lsd, but the condition would be unknown and the cost of buying plus rebuilding would get you near the price of a new truetrac. Talk to Draven on toymods, he got me a truetrac for $700.
There really isn't a cheap way with lsd's. After you buy it, it's a good idea to replace the bearings in the carrier and pinion and they are not cheap. Also, you should have it set up by a diff joint unless you know how to set the backlash/alignment.
Oh, and i'm lead to believe that the McLaren F1 road car runs a torsen diff system.