You are at the bottom of our failure. I think you mean: "you are the reason for our failure." You could perhaps say, "At bottom, you are the reason for/source of our failure." I bought a flower on my bottom dollar. I think Andrew is right that this is a very restricted phrase. Try: "I bought a flower with my last dollar."

He went to the country to take a break from big city life. The film is approaching the end. She married him for the sake of (making) money. You are at the bottom of our failure. - What's your intended meaning here? I bought a flower with my bottom dollar. (the 'bottom dollar' is not very used, other than the classic "bet your bottom dollar"