Try a compounding pharmacy.
My pup had to be on Doxy for 21 days last year for a tick related illness. I had to go to a compounding pharmacy to get it (liquid form) and almost crapped my pants at the price. My vet did warn me though. My vet also said that she understood the price increase and shortage was due to the manufacturer being wiped out by hurricane Sandy.
Hope your pup gets better!
Rompin'Stompin'RagtimeJazz - "Jazzy"