Murph had Hills ID too, its a very bland food which is very gentle on tummies and very easy to digest, but your right its quite pricey. He was on it for a few weeks after the giardia cleared up. I gradually changed him back over to his original food. For treats I used to get a tin of ID cut it up into small pieces and cook it in the oven until it was crunchy.
I still tend to keep a small bag in for Murph in case he has any diawotsit emergencies as I find it really firms things up. I've never been able to claim for any prescription food on Murph's insurance and he is with Petplan, but when I've claimed for Indy the Kennel Club Insurance pays 50% of the cost of the food which is really good.
Indy my other WCS had campylobacter as a pup and after we got that cleared up he was still having problems even after 2 negative results. He was then blood tested and it appeared that he was allergic to beef, pork, lamb and cows milk, the vet thinks that this was brought on by campylobacter as he had had no problems before. He's now on Fish4Dogs and has no problems, if he gets any tummy upsets he gets Prokolin and Fortiflora probiotic sachets on his food.