I don't want the high fiber food to cause a problem with kidneys when she doesn't seem to have one so far!
Unless your cat already has some degree of kidney dysfunction, I would not cut down on the protein content of food, since older cats need protein to maintain muscle mass. Has your cat had bloodwork done to check kidneys? If so and all is well, the amount of protein in this food is probably fine.
As for cost, prescription diets from vets are very expensive, where feeding canned food like Friskies would cost about 0.44/per day. Can't get much cheaper than that!