I paid $1500 ish and went with who I think is the best,Mike Jefferies.Best money I spent on the car and now have a world record because of him.Guzzi Mark
He told me $1200 for a race head about 5 or so years ago. I'm convinced that he is the best head porter that there is for a slant. I've seen what some of his work has done on /6 cars and it is nothing short of amazing.
I did mine myself and it was tons of work just for the porting. I have not gotten it shaved or had larger valves installed yet.