Hmm I’m not sure. With Discover magazine, it was one time, but a circulation of how many thousands or millions? This 500 copies on the CD seems pretty paltry in comparison though of course there’s the web exposure on top of it. You’d probably want to stipulate the size of the web version, but beyond that I’m not sure. Congrats on the opportunity, though!
you might also want to put a clause in the contract that would get you an additional bonus in the event that the cd is reissued down the road by a bigger label in bigger numbers, or something to that effect.