why does a 3½x5 cost the same as a 8×10?
I can think of two valid reasons:
— It’s the same sheet of paper and the same time to print; even if sending to a lab the effort is the same, though the lab may charge more for a larger print.
— To discourage smaller print sizes. Little prints get hidden away in a book, or a drawer. Big prints look better and are more apt to be framed.