Cob is free. Or you could do adobe, which is just bricks made of cob. Or cobwood instead of cordwood.
I'm not good on US geography but if you are coastal NE, you can make insulation out of eelgrass which is free. I think there used to be an industry there which made insulation out of eelgrass. Cabot's quilt I think it was called or something like that. There was some old 17th century house in cape cod that had eelgrass stuffed in the walls & it was still in good nick. If you live in a sheep farming area, you could buy a bale of raw fleeces & wash them & use them.
A friend of mine has a mud brick house. All the floors are clay. They tamped them down forever & then finished with linseed oil & beeswax ( I think ). Very nice. You can make clay tiles the same way as you make adobe bricks & use these.
Not sure about free roofing. Unless you can find a supply of thatching reeds, but then I doubt you'd get a permit for it or insurance on the building when it is completed. You can get roofing iron from demo yards but you would probably need to patch it.
You could volunteer to demo a building for the bits. I have a friend who got the flooring for his house dirt cheap by removing it himself.
Obviously demo windows & doors are still way cheaper than buying new.
http://www.thatroundhouse.info/ is an interesting cheap house too.