Its normally ~50-60 quid but it depends on how long you go for it can be cheaper to park your car there.
I've used two services, teh first was just off Sharston link road so ~5 -10 mins from teh airport. They drop you off in a mini bus its about £60 for this. The second was a bit dearer at ~£75 but they meet you at teh airport, take your car and bring it back to you. We preferred this as it meant Siobhan had her Car seat for teh whole journey and it was very slick indeed.
Google for car parking manchester airport and work through the links
Just realised I didn't consider if you didn't have a car or wanted to get leathered on the plane on teh way back
