I don't have a cat, but I live near a college the has a free pet clinic at their veterinary medicine school. The clinic also provides basic pet meds such as antibiotics for free or at cost. My little dog just had a tick removed from his ear, along with a test for Lyme disease and a month of antibiotics. I paid nothing.

If you live near a college/university. It may be worth checking to see if they have a clinic.

Hope your cat is feeling better soon.