He goes through (roughly) one of these per month, so that's $100, but he also gets it mixed in with this which lasts about 2 months. So about $115 per month which actually isn't terrible.
The real issues are the vets fees. Everything is more expensive when you're the size of a small horse.
My family and I also use royal canin. Tho they sell it in local pet stores and you can get a subscription to it on amazon if you have prime.
ROYAL CANIN SIZE HEALTH NUTRITION MAXI Adult dry dog food, 35-Pound https://www.amazon.com/dp/B000GQ414Y/ref=cm_sw_r_cp_api_YU3DAbD5DCR78
ROYAL CANIN SIZE HEALTH NUTRITION MAXI Puppy dry dog food, 35-Pound https://www.amazon.com/dp/B00CITLD2U/ref=cm_sw_r_cp_api_lX3DAb29GAWVM