It is usually cheaper in the long run to feed the proper diet than to deal with vet bills for not doing. Feeding the proper diet may not prevent office visits but it helps. Something like it’s better for us to do maintenance with our health rather than ignore it and end up in the hospital.
You may want to ask your vet if there are any coupons or discounts for the food. Explain that you’re trying to work with your budget and it would help greatly if you could get some kind of coupon or discount.