Is 400 a square to tear off and replace a roof a good price With 40yr shingles?

You're kidding I hope. Price varies quite a bit depending on where you are and the type of roof you have. One or two story, how steep, how many peaks, but that seems way out of line. 40 year shingles are probably around $100 a square, maybe $125. Single layer tear off should be half of installation and reshingle used to be about the same as the shingles to maybe one and half the shingles. If there is only one layer on the roof now, there is really no reason to tear off what is there. For that kind of money, I would get several bids.