Our club has used at least 3000 feet of Gargraves flex track, likely much more. Lay it out as Mike suggests with a homemade compass, either a string or wooden stick. Then bend gradually to follow the curve. The waste generated by the inside rail having to be cut even with the outside rail won't come close to the cost of pre curved track. I believe Gargraves flex is about the most economical track you can buy.
Pete