The quaife LSD for the 6 speed getrag box from the 170 is IIRC about 25% more expensive than an LSD for the MTX75 which is found in the 2.0 focus. (probably around 1000 quid).

Someone here told me once what an FRS gearbox cost new, I'll search my posts and find out, but that's the box you want, cause it's an MTX75 with the quaife drive. Alternatively, you could get an MTX75 box from a parts dealer and buy the quaife LSD for it and fit it.

Sorry, can't seem to find the thread with the information, and the search feature is broken, but I remember that new from Ford the MTX75 was about 890 quid, and the FRS box probably closer to 1500 ... but I hope someone that knows for sure will correct me if I'm wrong