What's the best and most affordable food for my St. Bernard puppy?

Saint bernards, as will all large and especially giant breed puppies have very specific nutritional needs as they are growing and developing and a well formulated diet can help to minimise the risk of developing growth and joint issues such as hip dysplasia - I would recommend royal canin giant range up to 18-24months of age, feeding strictly to maintain a healthy body condition score (lean but with adequate muscle development) in order to optimise healthy growth. A cheaper alternative would be a greyhound puppy food but this is not as specific for giant breeds. A saint bernard is not an 'economy' breed of dog and remember that all routine treatments, insurance policies and medical/surgical care are going to be significantly greater than a medium sized dog throughout their life