Back when I was college (and that was a while ago now), the 2nd or 3rd year CS students would grade the 1st year CS class (which was required curriculum for all 180 students across all of our technical majors). While we did get paid to do so, it wasn't a lot. There was a strong cultural norm that doing "grutoring" (grading and tutoring) was just what you did.

I suspect that utilizing near peer style grading would be fairly scalable.