kit car peeps tend to pay the most for 7" diff's and can be anything from 100 quid, however the 3.92 LSD read diff isnt overly common as it fitted to those 2 models, the V6 4x4 LSD is far more common however is 3.62:1. for a good condition 2nd hand LSD diff i'd pay between 50 and a 100.

If you're rebuilding a diff anyway the easiest way is to use a V6 LSD casing and use almost any Sierra diff crown and pinion, i do have a list of models with ratios somewhere but i'll have to dig it out, however alot of base model sierra's came with 3.92:1 ratio.

Value tho is a funnjy old thing in the 2nd hand market, how much is something worth? My opinion is something is olny worth what someone is willing to pay