Brunch will cost whatever you want it to cost.

Mainly you will just need to be sure you meet the food and beverage minimum at the location you choose. Your wedding planner will give you menus as a starting point, but you can completely customize them to suit your taste and budget.
To get an idea of the options and the prices of the set menus, check out this website:
http://magicalkingdoms.com/ftw/wdw/menus.html
Disney's official website has mostly general info. For real details and prices, you need to check out the various independent websites, books, and message boards devoted to Disney Weddings, most of which turn up in a simple Google search.
This board is an excellent source of info from real brides, and you might enjoy reading through the planning journals and trip reports found in the subforums listed at the top of the main page.
For example, if you click the "PJ" link in my signature, you can read about how we planned our brunch, what was on the menu, and what it cost. The wedding report, with pictures, is under the "Wedding!" link in my signature.
Welcome!
