how much to replace heat blender door

Hello, what you are asking does not actually indicate a part and labor. The door itself is part of the plenum, which by itself is not repaired separately. It sounds like you have a problem with mode door operation either with the input data or the motor that controls the door movement. You could also have a fractured door or plenum, which would require expensive part replacement. Hands on inspection will be needed to find out what needs to be repaired. Use this link to find some shops near you that can help with your concern: www.repairpal.com/repair-shops