i was lucky with my gear box as i got one from a friend who happened to have one cause he used Zetecs for rallying and had a few spare box's! (do u know how hard u have to haggle with a man that keeps gearbox's for rallying???)
i paid £800 for mine and that came with the quaife LSD in it

think quaife do a full box conversion for £2500 with all internals straight cut and LSD fitted. check out their website for a catalog that gives u all the prices depending on what kinda box u need