What makes sails expensive except for the materials is the time the sailmaker use when building it. For a radial sail the sailmaker will have to cut all the panels, glue them together with double sided tape and run the seams trough a sewing machine. This is a time consuming process requiring meticulous handwork for the sail to look good. The rest of the job, fitting luff rope, edge tapes, batten pockets, reinforcements and grommets also takes time, but is the same for the two technologies.
With tapedrive and similar technologies, there are not as many panels to glue together to make the shape of the airfoil. It looks like just 5 panels in the Mozzie sail Chris posted pictures of. Once the panels are glued together, the sail is folded so it lies flat and the sailmaker walks over the sail with a battery of tape dispensers. Or the process of applying the tapes are automated in a machine, like the North 3Dr machine.
Unless the cost of the materials are far higher for a "string" sail, they should be cheaper to build. If they are not cheaper, the sailmaker either have some investments in equipment he has to write off or he wants to make more money <img src="http://www.catsailor.com/forums/images/graemlins/grin.gif" alt="" />
String sails should be a bit better than radial cut sail with better shape holding properties, but it's the sailors who make the sails shine. I would dearly like to have a set of string sails <img src="http://www.catsailor.com/forums/images/graemlins/smile.gif" alt="" />