No-one is going to be able to give you an accurate quote for a job like this until the cause of the problem has become clear. It's extremely likely that the cylinder head will have to come off, but what's found after that is as yet unknown. Removing and refitting the head alone will be several hours work, and that's without having to go any further.

A work colleague bought a 4 year old Mini from an independent dealer.. 5 months into ownership engine maangement light came on, limp mode and dealer 'fixed it'. Problem reoccurred several times until dealer warranty expired. She had confidence in dealer but should have have rejected the car. Ended up with £1k worth of 'repairs' without success and finally gave up trading it in at a loss.