All food at OSH is the food tent variety, and around $9 or so like they said. Restaurants outside of the airfield are not really within walking distance, except maybe Friar Tucks (located just outside the airfield). The only problem with eating offsite is that the restaurants are extremely busy, so make sure you go plenty early.
There are plenty of ATMs scattered around the grounds, especially by the exhibit hangars. Definitely try to hit the exhibit hangars right when they open, around 10am, because the crowds can be pretty insane depending on the day. I also recommend taking the shuttle bus to the Seaplane base for a couple hours. It's a lot of fun to see the wide variety of seaplanes that come in and out, plus it gets you away from everything for a little while.
The museum is also free with admission and is a great place to get some air conditioning on the really hot days. Just be sure to stay hydrated and wear a hat/sunscreen because shade is limited. I love OSH with a passion and have been going for about 10 years now, with this year probably being the first year I won't be able to go. Still trying to see about getting some time off work. I used to live about 2 hrs away, but now I am a good 8 hrs or so which makes it more difficult. Oh well, enjoy your time there!