You obviously will have a lot of costs you haven't specified, which would ultimately determine your per serving charge (or be determined by it, if it is really fixed at $35), Have you thought through all your costs (product, labor, serving costs, venue, cleanup, etc? Really hard to answer your first question without those numbers. Could be serve 8 oz, could be you can't afford it. Any way I'm thinking you need around 40 - 50 lbs of prime rib. I don't know what current wholesale is, but retail for the good stuff (choice angus) is around $14 a pound.
No idea what your La Caja China can hold, but from the ones I've seen, that amount isn't going to work. Maybe others have better, more professional opinions.