My Cat has Diabetes, being Ripped Off?

My 6 yr old male cat was recently diagnosed with diabetes. I took him to the vet who hospitalized him for 3 days measuring his glucose e.t.c Since bringing him home he seems to have worsened if anything. I am giving insulin once a day and due to take him back in a few days for another blood test. So far the vet has charged �400 or $750. And everytime I take him for another blood test he wants another $60. I can't afford this. Can I treat the cat myself without the need for a vet??Also, can I feed him tinned tuna as part of a healthiy diet, as these low carb dietary foods cost a fortune too?? thanks anyone