Some smaller labs at my tier 1 research institution have their grad students TA (so the university pays their salary) because they don't have the ability to pay them the $30k salary. $11k for one paper is a meaningful amount of money, and other journals can publish for an order of magnitude less.
It doesn't cost $11k to read a paper, send a few emails and do some typesetting. People pay for this kind of thing because having a Nature paper on your resume makes it much easier to get a tenure-track job. It's just a prestige racket.