Dog Food Veterinary Diets ?

My great dane puppy was switched to purina veterinary diets for large breed puppies due to allergies, It costs about $60.00 a bag, and he goes through just over a 35lb bag a month at 6 months already and he will go through almost 2 bags a month when he is full grown, and I was wondering if anyone knew if there was a "cheaper" dog food i could mix it 1/2 and 1/2 with (he is allergic to pedigree) that has similar ingredients?