Nova is the cheapest you will find for main tubes. They are good tubes and I've built several frames with them. I've found that straight gauge 4130 tubes cost the same or more than the butted Nova tubes.

I use straight gauge .035" tubes for seat and chain stays, because I don't like the look of tapered stays. I can bend and shape them to suit the build and don't have to worry about cracking or buckling a heat treated stay, from trying to bend it.