They may need to drop the K per their shop manual. Going rate around me is $90 for a non-dealer labor rate. Best bet is to call around. Headers in the 4.6L suck a LOT to do with the motor in the car.
Don't be surprised if its a $300+ job on a 4.6L...
2000 Mustang GT Steeda build #48. Bolt ons, suspension, Novi 2000.
Just because I give you advice, doesn't mean I know more than you. Its just means I've done more stupid ****.