Actually, flexible line is cheaper, when you consider the fittings, hangers, possibility of leaks and then having to dismantle, and installation labor. It is just as good if not better than black gas pipe, as long as you don't damage it, and that is fairly hard to do. Observe the particular manufacturers bend radius requirements, and you don't need 45 and 90 fittings to make a turn.
If the installer has a fitting leak when pressure testing, he cuts the line at the fitting, reinstall the line and new clamp on the fitting, and squeezes on the clamp. Less than a minute. Try that with hard line. Some gas line still require compression termination fittings, but it still much faster and thus cheaper than metal pipe.
