My cat had diabetes and I used to get syringes at Walgreens - I think it was about $16 for 100 but this was 8 years ago. You don't have to have a prescription to get them, so call around to different pharmacies and see who is cheapest.
Regular insulin (Humalog) was not too expensive, but unfortunately it didn't work for my cat. The vet had to order special veterinary insulin - the vial was $66 and lasted about 2 months, as I recall.