Animals are at least an order of magnitude more expensive in pure energy inputs to successfully grow than veggies (this becomes apparent if you compare land usage between the two purely in solar wattage).
They're able to provide much more energy as a food source, too. That means we'd have to eat several pounds of veggies to replace the energy we get from one pound of meat. If you're going to compare the cost of growing animals vs veggies, you've got to account for needing more veggies.
It's a complex equation because there are a lot of factors that have to come into play to do a fair comparison. It's probably as complex to model, and as sensitive to input-variable selection, as predicting global weather patterns.
It's not really that complex. Due to the 2nd law of thermodynamics (entropy), you will never get more back than you put in (in fact, you'll never even break even).
Animals are at least an order of magnitude more expensive in pure energy inputs to successfully grow than veggies (this becomes apparent if you compare land usage between the two purely in solar wattage).