Generally parking and general admission are free and you can walk around and watch divisional games and attend any of the booths and tents for free. Tickets to the grandstands on the pro fields are usually inexpensive. $10-25 and less for kids.
The only thing you may want to consider budgeting for other than food and beverages is discretionary - if you want to pick up any gear at any of the vendor tents or often pro players will sell jerseys and gear at the events and you can buy it right from them. Cost depends on the item, a signed jersey might run you $150 on the spot.
At World Cup NXL had elephant rides, volleyball, and other secondary entertainment on hand as well. I'm not sure whether they charged for any of that.
That's about it.