The final cost will depend on how long it takes, hourly rate and parts used.  Book time is about 5 hours.  It can be done cheaply at Bob's Back Street Bodge shop using EuroCrapParts stuff for less than £1200.  Is your car tuned?  If it is, AKS are doing the Helix upgrade (with DMF) for £810. Plus 5-6 hours @ £62.50.
You definitely won't get the job done for under a grand with quality parts at a reputable garage.