How much is it going to cost me ?
That would depend on the volume of work.
Personally I think you should give cleaning it up yourself a bit of a go first. There are tools to help you out - if you have Firefox the web developer toolbar has a number of features that will help. For orphaned/unused rule sets the Dust-me Selectors extension will help. If there's anything you're not sure on I, and no doubt others here, will try and point you in the right direction.
Of course, if you don't want to have a go and do it yourself then fair enough, consider paying someone.