There will be advance sale opportunities along the way for Summerfest tickets, but if you end up buying your tickets at the gates those 10 years of age and older should expect to pay $15/ticket if you enter after 4p, with kids and seniors asked to pay $3/ticket. If you enter between noon and 4p, admission is discounted. Children 2 and under can enter free of charge at any time.
As Summerfest dates near, there will be discount offers and promotions offered to save you a few dollars.
Again though, with your Roger Waters tickets or Bon Jovi tickets or any Marcus Amphitheater tickets, you won't need a separate admission ticket for the Summerfest grounds.