Admission Budget: $0 Seriously, do not pay for admission.
Parking Budget: $5 You'll have free access to all the stuff mentioned above, but since you have a little cash to spend, let's get you to the fair on the DART, which'll be $5 (unless you're old, in which case it's only $2.50, because half-off is apparently a reasonable discount for the still-not-dead).
Food and Rides Budget: $5 Buy 10 tickets and spend them on the fair food or ride or Midway game of your choice. On Tuesdays, rides are half price, so a couple times around the Puke-A-Whirl might be fun, especially right after a dollar hot dog, available on the patio of the Old Mill Inn. Two things about those dogs: 1. They're real hot dogs, not a carnie's invitation to a very public blow-jay party. 2. I'm positive they're not the best food item at the fair, but they are cheap and passing out at the fair from lack of foods in you would be a bad thing. So if you're in a tight spot, dollar-dog it up.
Admission Budget: $0-$5 Either go on a day when you can get in free or, if you love riding rides at the fair, bring a Dr Pepper can and only spend $5 on a Tuesday for admission. Every Tuesday, rides are half price.
Parking Budget: $5 You should definitely do all the stuff mentioned in the lower price-point options. Get all that free stuff, and absolutely ride the DART to the fair for $5.
Food and Rides Budget: $10-$15 With $10-$15, you can do all kinds of crazy fair stuff. You can eat crazy fair food, ride rides or play Midway games. You could go on a day when you get into the fair for free and then you'd have the money to buy a ticket to the Chinese Lantern Festival ($14 for adults, $9 for kids 4 to 12).
Spend the money this way: Pre-drink 18 Four Lokos at home for free, pay $5 and ride the DART to the fair (or put some Depends on your head and pay $2.50) on a free day. Then, spend the extra $15 on fried chocolate-covered strawberry waffle balls on a stick (three for $6), a ride on the Texas Star ($7), and use the two dollars left to bribe an old dude to tweak the nips of the Big Tex butter sculpture.
Alternately, you could always spend your money on America's favorite carnival ride: beer. I'm told the cheapest beer at the fair will be at BW's Fried Ribs in the Tower Building. If there's a long beer line, tell everyone Big Tex is doing a Channing Tatum-y one-leg-up dry hump of the Texas Star and watch your hops troubles disappear.