Hey,
Just had to have mine dropped in for a complete rebuild (crap overdrive on the box buggered it all up apparently), decided in the end I'd prefer to know the history of the box, and seeing as how i'm doing a new chassis rebuild, thought it will never be as easy to get to the box - so lets get it right now!
You can pick up a second hand use-able box for around the £250 mark, but bare in mind that this is a box just taken out of a vehicle in a breakers yard - no history, usually no more than a 3 month warranty either.
You can get a newly reconditioned box for a bout £450 I think (in the land rover mags), but that is just the 4 speed box, need to budget a transfer box onto that as well, from what I remember, I was quoted around the £800 (+ delivery) mark for a complete newly recon box - on a part exchange basis for my old one.
The guy who is doing my rebuild says the trouble is finding decent parts to put in them, so you want to try and get a decent recon if possible, if your budget will allow. Hopefully trouble free motoring for a few miles then..
Matt