Lowest cost of proof: Amazon
My book comes in at 244 pages, black and white interior with a color cover, and a proof is $3.77. Blurb and Lulu proofs cost more (these are regular books, not proofs), and while Blurb is a couple of dollars higher than Lulu, their shipping cost is prohibitive for single books. They also charge per book for the removal of their logo.
Best color on interior pages: Lulu
For all three companies, white is a bleached white. Lulu has the best cream, and Amazon’s is the darkest.
Best cover: Blurb