User blog comment:Belthazar451/Number crunching: Food, Glorious Food/@comment-5832451-20130524041917

Lots of mathematical pseudo patterns can be gleaned. But the most practical advice is that the cost of of advancing from Level X to X+1 is basically the same (it's actually higher) as getting a new creature from Level 1 to Level X.

So if you have 2 Entbrats, one at level 8 (A) and one brand new (B), it'll cost slightly less food to make Entbrat B level 8 than it will to feed A to Level 9, and the difference in earning rate and maximum (and consequently the Return on Investment) is FAR higher on B.

Early on, before Gold Island came out, there was little reason financially to make any creature level 15, at least until you had fully developed islands and were just looking for ways to spend money.