Depends on what you wanna do. Your cylinders have to come off regardless. If you are getting a drag port, sometimes the builder likes to port the cases. I recently had my drag style porting done, cost me $500 for the porting of the cylinders and $100 for the porting of the cases.
There are alot cheaper options out there. Just really depends on what kind of porting you want and what kinda of riding you do. I used to have Eric Gorr's woods port that cost me $225. I liked it alot. Had a ton of midrange power. What kind of riding do you do? Drag? Sand? Woods? Track?