Does running several graders on one trace cost more than running one?

Not proportionally, if the graders share a prompt cache. A cache write costs 1.25x the base input rate and a cache read costs 0.1x, so the first grader on a trace pays to write the input to cache and every grader after it pays a tenth of the read cost for that same input. Run N graders on one trace and the input cost comes out to roughly 1.15 + 0.1N times the base rate, against N times the base rate if each grader paid for its own copy of the input.

At two graders that’s already cheaper than uncached; at ten graders it’s about a fifth of the uncached cost. Break-even lands around N ≈ 1.28 graders per trace, so a single-grader check is usually left uncached.