Thinking about purchasing a diff for the Fiesta ST, as when your going round a roundabout you get severe inside wheelspin and it loses you so much grip, hoping this will help...

My question is, how hard is it to fit a diff to a gearbox that doesnt have a limited slip? ie

http://www.pumaspeed.co.uk/showdetails.php?id=320

How much would someone charge to fit a diff to a gearbox? is it really expensive?