I mistakenly thought the rigid liner would be significantly cheaper but it appears that the HeatFab rigid 304 (which we put in at my BIL's) is now pretty close to the same price as the Homesaver Roundflex 304 that I have here. In that case, I'd go with the flex..you don't have to screw sections together. The cheapest flex I would consider would be a medium-weight liner such as HomeSaver Pro at .006" (not UltraPro.)
Homesaver pro is light wall also the midweight stuff is usually .010