As
@hoopity implied, petfood only has a diamond value when one is depleted. Absent that, IMO the diamond value is what gamers will pay in order to stock up on an already existing supply.
This may be beside the point because I infer that you are looking for a cost equivalence within the existing cost structures of resources but, like you, I can find no relationship between petfood and existing resource costs.
Therefore (at the risk of being bombastic) the diamond price of petfood is empirical and not relational.
Empirical may be true but only in a non competitive format. The supply of pet food does not fluctuate so there is, at some point, an empirical value to it. This value holds not because it's intrinsic, but because there is no competition for the resource. It's sort of like air on the planet Earth. It's everywhere, it's in demand, but the supply is far and above any level of demand that might be envisioned. So there is no competition (except in rare Hollywood imaginations), and thus little to no fluctuation in it's value.
On the other hand, supply may be steady, but demand isn't. Thus, to the individual trader rolling the crafting over and over may be a good deal if they are short pet food. It's like if you are trapped below deck in a sinking ship. You might fight very hard to get that last gulp of air before attempting to swim your way out. And you might even do some physical competing with any one around in the same boat (pun intended). Thus, air, in normal circumstances, has a small to non-existent value, but in special circumstances is worth as much as life itself. Lacking pet food may not kill you, but when you don't have it, you might very well spin spend the crafting wheel more than you would if you had even a little of it.
So what is being asked is not the value of pet food, but it's comparative cost against other resources. The game makes that an empirical calculation since we don't compete for pet food. On the other hand, cost, as a consideration of other goods where there is competition and fluctuation in supply/demand, is important but not the whole story.
Making the distinction between cost and value is important as it's been neglected in the past and some people have had a hard time re-considering what they "pre-considered" about the matter.
AJ