It sells at the $30 range because the minimum price which you can sell to the private market is initially $29. When you have some military tech, demos can bring this up to $36, hence that is the floor price of food.

Basically, food is produced in such plentiful amounts that it will almost always be near the $36 floor price, rather than the $261 ceiling price.

And part of the thing is that based on the current game equations, food at $45 is actually equal income for a casher and a farmer at the same land size (both with appropriate maxed techs, and equal army size). If the equilibrium point was a higher food price, then more people would play cashers over farmers (since cashers would perform better if so), resulting in less overall food, and price of food would then raise to the new equilibrium, and vice versa.

As it is, food will average $45 as a consequence of the game equations, going above and below it in equal measure over the length of a reset.