If you have a hoist and are competent than rebuild it yourself, cheap for bits. However its a gearbox out job, obviously, so a garage you're looking at at least 6 hours labour...so thats £400 at least.
I'd look around for a good box on here or on ebay (£100-£150) and just do a swap. JamesMartin (member here) may be able to help with a box.