Summerfest is one if not the largest music festival in the world. It also just so happens to have a great range of breweries and beers available to consume each year. While there might be some missing you can count on finding Miller, Leinenkugel, Coors, Lakefront Brewery, Water Street Brewery, and lots of Miller Lite and Miller products. I may have bought a can of Old Style off a girl for $2 a few years ago but I think that was the only time (it seems she had brought them in with the band and then decided that she wanted to sell them – we gladly took a few off her hands).
Beer prices depend on what you want. For some of the craft beers they are a bit more but still typically around 6-7$ while a Miller Lite or Coors is more in the 5-6$ range. Not cheap but not sports stadium prices either.