I've never had to give any of my dogs this drug. However, when I was buying drugs for my bridge girl I bought from Sam's Club. I don't know if you have any of these in California but I'm betting you have some form of warehouse club that offers discounts on prescriptions. Check them out. Good luck. I'm sorry your pup has Cushings.
I don't have a dog with Cushings. However, for most prescriptions Foster's and Smith or Petmeds have the best prices. What I have also seen with Petmeds is if you find it anywhere on the web for a price lower than theirs, they will match the price.
CostCo has the best price on prescriptions in our area. They are about half the price as other sources. The only thing is they only carry the drugs that people use. If it is a Canine drug only, they will not carry it.