The first ingredient being the most and the last the least, the first ingredient should always be a form of meat other than fish. Ideally all 3 would be meat, although there are very few that are. Fish in general would be better to avoid, first because foods that include fish high on the list of ingredients are most likely designed with minks in mind not ferrets, another reason you may want to avoid fish is that ferrets that eat foods with a high fish base tend to smell like fish. Corn should not be one of the first 3 ingredients. Actually it is best to avoid corn in all forms altogether.
Corn is hard for the ferret to digest.
Another thing to look at is what form is the meat. Ferret should only be fed dry ferret or cat food, an occasional treat of a spoonful of canned ferret or cat food is ok but not on a regular or frequent basis. Unfortunately cost does have to come in as a factor, spending top dollar on the most expensive ferret food and not being able to afford medical care when the ferret needs it is pointless. Feeding your ferret the most expensive food is not going to prevent him from becoming ill. However feeding your ferret a quality diet will lower his chances of having certain problems in the long run.