I could also be missing something important in my model, like if the way we subsidize corn makes it basically free to feed to cattle, but still costly to feed to humans.

I wouldn't be surprised if transportation + milling + packaging make for a lot of this difference. I would expect bulk meat prices per unit to drop more slowly than bulk grain prices per unit, which is in the right direction to turn a 1:40 ratio into a 1:10 ratio.