Its always cheaper to get a prescription and then buy online, although vets do charge for the prescription. remember you will also need to pay for blood tests and check ups every 3-6 months.
If your cat is otherwise in good health and not too old then I would strongly advise looking into either surgery or radio active iodine therapy as the ongoing costs of medications do add up and the drugs do not cure the disease merely aim to halt/slow it's progress.