Depends how old it is, if it's non transponder key it will cost about £5, loads more if its newer and has a transponder immobiliser.
Fobs are not that cheap but you can get a used one programmed to your car at rover dealer or look for locksmith that does coding in the yellow pages.
Edit:
It doesn't HAVE to be a dealer. Depends how much you want to spend.
When i had a unit at weson super mare we used to get a guy in from Clevedon who would programme the fobs to the vehicle for £35, fobs used to come from scrap yard so for £40 you can have it sorted if you if you phone around.