At the moment, all these are anywhere from free to two bucks. The Darwin books are always cheap, the others are probably temporarily cheap.
If you've not read The Autobiography of Charles Darwin, you should. It is always avaialable for next to nothing on the kindle, currently this version is 99 cents.
Concerning his autobiography--written when Darwin was 59 and originally published as the first part of “The Life and Letters of Charles Darwin” (1887)--Darwin explained: “A German editor [wrote] to me for an account of the development of my mind and character with some sketch of my…