A BTR (BW) LSD has a warranty life of 100 000kms, these days. Furthermore, the centres can no longer be shimmed to allow for wear. The minute an LSD doesn't grip, it begins digesting itself quite quickly. A good "setup" is desirable. Expect to pay $150 for checking, assembling and installing a good centre into a diff.
As a guide, I paid $250 for a 4P LSD with adjustable cones. The cones were shimmed and side gears second hand; everything else was new.
When a centre is professionally installed, the backlash and preload are set. These are not arbitrary figures, but design parameters to give good life and performance from the part. I have not yet heard a good argument for ignoring them.
Cheers, Adam.