To be honest, puppy food(I'm assuming you mean that) isn't cheap but it gives the pups the best start. They dont eat much, so a 15kg bag as Anita says should last a while if you only have one pup. Bulk buying is ok, but you have to store and use it within dates, so ok for a lot of dogs
I use skinners...its reasonably priced and my gang seem to do OK on it