Affordable substitute for cortisol injection for my dog?

I have a 3 year old chihuahua girl. She has very sensitive skin, especially if it comes to flea bites. Unfortunately we live in a hot area and there are a LOT of fleas. Even a few flea bites make her itchy so badly that she scratches the areas until she draws blood. She also loses a lot of hair due to the skin... show more I have a 3 year old chihuahua girl. She has very sensitive skin, especially if it comes to flea bites. Unfortunately we live in a hot area and there are a LOT of fleas. Even a few flea bites make her itchy so badly that she scratches the areas until she draws blood. She also loses a lot of hair due to the skin irritation.

We took her to the vet. He could not find anything physically wrong with her. He recommended getting her a high quality diet as well as bathing her frequently and gave her a cortisol injection to soothe her itching and help her skin recover. We are feeding all our dogs "Natural Balance - Original Ultra" - so the quality diet should not be a problem. Other than that the cortisol injection actually helped a lot. Only a few hours after the injection we noticed that she scratched way less and within about two weeks her skin started healing and she stopped losing hair.
Unfortunately about 3 weeks after our vet appointment the injection stopped working and now, 6 weeks after her injection, her skin looks as bad as before. The cortisol injection cost me about $35 - which brings me to my question:

Is there a more affordable way to get her a cortisol treatment?
Thank you guys in advance.
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