Estimating the costs of a vacation or long road trip is important (unless you're Bill Gates) but it can be challenging, and costs can easily get out of hand if you don't plan for contingencies.

Road Trip Major costs

• Gasoline - by far the largest cost, if you are camping and going any distance.
  But... there are things you can do about it. Use the GasBuddy app to find the cheapest gas on your trip.  If you have memberships to COstco or Sam's Club, they usually have the best prices and are usually located near Interstate highways.
  If your car or truck is a gas guzzler, it may be less expensive to rent a more fuel efficient car or truck, especially when you add in maintenance and depreciation.  See this page for "drive your car or rent a car" calculations.
  Worst case scenario: take the total miles of the trip from the mapping software, divide that by your car's city MPG and multiple that by the national average price of gasoline.
  Example: Honda Pilot, 18 mpg.
• Camping or Lodging - Obviously, camping is much less expensive than motels, hotels, B&B's or lodges. In any case, getting reservations in advance, and shopping around can get a better rate.  Having the National Parks Pass also gets a discount in most NPS campgrounds.
• Food - This can be a minor cost - or a major expense.  It's up to you.  Restaurants can easily be 4 times the cost of making your own meals. If you take a cooler and buy food at local grocery stores, it should cost no more than eating at home. Stick to simple meals see recipes here - and you'll be fed, happy and it's easy.
• Admission prices and other fees - If you are going to National Parks or monuments, be sure to get the National Parks Pass. Private amusement parks, like Disney World or Six Flags can be very expensive.  Annual passes there, too, can be worth it for multiple day visits.
• Tolls -  Toll roads can be an added unexpected expense, but most mapping software can show you how to avoid them, although that may add time and distance.

Ballpark estimates

As a rough guide for long trips, figure $50/day for gasoline, $35/day for campgrounds, $15/day per person for food (assuming you prepare your own meals).

So, an average day for a family of 4 should cost about $150, pretty much all inclusive, except for admission fees, souvenirs, etc.