5 1/2 Year old male boxer with elevated liver enzymes, vet suggests Hill Prescription LD dog food?

Axle went from being perfectly fine to barely moving in 12 hrs. I rushed him to his vet and found that his liver enzymes were close to 2000. He was hospitalized over night and sent home today with meds for his liver, antibiotics and 2 IV bags of fluid so that I can give him 120 mL in the morning and at night. He has to return to the vet on Saturday for another xray (the first was not clear) and more blood work. To my knowledge the only change in his life style was his food, he had lost a bit of weight so I purchased Purina Dog Chow with high protein. The vet is at a loss for why he is having these problems but that is trouble some to me. I am asking for suggestions on 2 things, first why might this be happening IF (hopefully) the xray comes back okay? Second, the prescription dog food that my vet suggest is very expensive close to 60.00 for a 18 lb bag are there any other dog foods on the market that are a bit less expensive yet still as affective? Thank everyone in advance!