Convex machining vs concave though? I don't know for sure though. Just a wag from watching wood cnc's go at it. The feed rates in wood are faster, but there are some crazy fast metal machines too.
Its hard to say, there would be a rather large amount of metal removed and that means many many passes.
Trying to build a foil from sheet metal with well designed and stout spars would be lighter and I could imagine them being strong enough, still not sure about it being much cheaper though.
