The lines needed for traction kite flying need to be a lot stronger than 'standard' kite lines for trick flying - simply because traction kites are designed to pull you along so therefore need to be able to support your weight. Usually the top, or power, lines are a stronger weight than the bottom, or brake, lines as these are the lines that take most of the strain.
I have a single 'live' set of lines (along with handles/kite keepers) that I swap between my two traction kites - when I add further traction kites to my collection I will probably add more lines sets to save having to change lines over too much. These are Dacron, 30m long and are 170lb breaking strain for the top/80lb for the bottom - and have lasted since December 2001.
Cost: Depending on length and breaking strain around £15 per pair. All kites come with recommendations for line length/strength (if indeed they don't come supplied with lines!).