new box A LOT!
2nd hand box for an 899cc cinq will cost about £70 delivered,theres one on ebay now.
2nd hand box for a 1.1 sporting will cost you probably about the same ,theres a guy breaking one on here ,look in the classifieds,his gearbox looks mint!
also when changing a gearbox you would be better changing the clutch
if you are getting a garage to do the fitting of the gearbox i would say about 1-3 hours labour ,about £65 per hour