I would recommend that you attend some dog shows in your area. At the show you will be able to see each breeder's dogs and how they compare to others in conformation in the show ring. You can also pick up a program there which will contain the names and advertisements for various breeders with pictures of their dogs looking their best.
Though I'm not familiar with Minnesota, I would assume that the price of puppies, like everything in our economic system, is to some degree affected by the laws of supply and demand. Breeders with more winners and champions can ask higher prices because their dogs are more in demand. Breeders with consistent winners in their bloodlines are also breeding for specific conformation and their puppies, whether "pet" or "show," are likely to be of higher quality than those of an inexperienced beginner or backyard breeder. Many times with established show breeders, they pick the buyers for their puppies because there may be a long waiting list for an upcoming litter. It's almost like an adoption; you don't just get to pick any puppy from the litter.
Reputable show breeders will also have done all the necessary health testing on their bitch and stud prior to breeding to insure the best chance for healthy puppies and dogs with as few health problems as possible.
Check out this website -
http://www.worldwideboxer.com/
it contains lots of valuable information on the ideal Boxer conformation and may help you in your search. Good luck to you. There is nothing like a wiggly Boxer puppy to make you smile and bring you happiness -