I did something similar (to pass inspection they had to move the existing gas line over one joist). I believe it cost about $1,500 but I am not sure how much of that was for the kitchen line and the moving of the existing line. My stove sits directly above my gas line so all they had to do was extend the main line up about 3'. It was the most expensive individual thing I did in the kitchen renovation.
A lot will depend on where your existing line runs in relation to your kitchen./stove