I'd imagine just a Mac and Xcode would be acceptable to start. Xcode will have all the developers tools you need, and is free to download from Apple (link: http://developer.apple.com/ , it'll take you to the app store to download ). The Mac could be any machine with Lion, and we got out Mac Mini and a 23' display for about $850.
Short answer if you don't have a Mac yet, $850 + an internet connection.
Xcode will have simulators for the iPad, iPhone and iPod, so you won't need one of those to start, but may be something you add later. An iPad runs about $400.