BobG said:
$1500 is way high if you're traveling when the weather's warm. Camping is a good option which also gives you the option of cooking your own food (Coleman stoves are pretty cheap and work well, as long you make sure to have an extra bottle of fuel). Plus, if you don't know where you're stopping that night, then you can't make reservations and you'll wind up in motels where $50 would be the highest rate you could expect.
You could make it on $750. I'd have a good credit card as insurance, though. If your car breaks down, you don't want to be stuck in South Dakota with no way to fix your car.
It would depend on the length of your trip. As suggested, you can camp out and cook inexpensively, in which case, your meals won't cost any more than if you were still at home, so that's a negligible expense (you'd have to eat whether on the road or not). If you have to stop and eat in restaurants every place you go, then it's going to add up to a lot very quickly. I'd suggest packing a good cooler (use ice blocks instead of cubes to last longer on a long trip) filled with sandwich fixings, some fresh foods to last a few days at a time grilling dinners, a bunch of beverages, including plenty of water. Along with that, pack up some dry goods...breakfast cereal, canned foods (don't forget a can opener!), toilet paper if you're going to camp along the way, baby wipe type things (good for cleaning hands and other body parts if you need to freshen up in the absence of water). This will keep costs down considerably.
Some truck stops even offer shower facilities, so you don't even need to find a campground with such facilities if you can stop at a truckstop and use the shower there.
If you're willing to sleep at a campground, on dirt with bugs, then it's also not such a big deal to spend some nights at fleabag motels...the goal of a cross-country road trip isn't to lounge around in luxurious hotels all day, but to just use them for a bed and shower and roof over your head on rainy nights.
As for planning just how much to save, I suggest sitting down and mapping out the path you'd like to take, figure out the mileage, tack on some extra for sidetracks off to see random roadside attractions you might want to see at the spur of the moment, assume you might get stuck having to buy gas at some expensive places because you can't make it one more exit for cheaper gas, and based on your car's fuel efficiency, you can estimate your approximate fuel costs (and tack on a little extra for driving through mountains or sitting in traffic, which will consume fuel faster than driving on flat highways). Then, plan where you want to stop. If you're stopping to see just little scenic spots that will only take between a half hour and two hours to visit, you can plan to drive 6 to 8 hours in a day (more if you have a driving companion) with those stops in between, and then a good night's sleep. Leave time for places that might take longer to see (i.e., an attraction with long tours that might take a day out of your trip), and then you can figure out how many nights you're likely to be on the road (again, add a few in case you just get too tired and need a day to rest somewhere, or run into mechanical problems and need a day or two to wait for car repairs) and you can guesstimate your food needs and potential lodging expenses. I'd err high on that, and assume the $50/night average (with some nights being free sleeping in your car in a rest area, and others being more expensive if you have to find a place in a touristy area). Better to have money leftover than to run out.
Plan that you'll want to get your car fully tuned up before you leave so you have less chance of breakdowns on the way, and include that in your budget...you need to keep your car tuned up anyway, but you're not going to be as flexible on timing to wait a few extra weeks for that tuneup if you're about to hit the road for a long trip. Make sure you have a good spare tire, jack, lug wrench, a few basic tools, oil and antifreeze, and any other little items you can think of that might make the difference between sitting on the side of the road waiting to pay for a tow truck on top of repair bills vs. being able to make it to the next exit to find a service station. Though, these are all things you probably should have anyway, so you can start acquiring them anytime and don't have to figure it into your vacation budget.
Whatever budget you arrive with based on fuel costs and food/lodging for the number of days you'll be on the road, tack on $200-$300 extra for unexpected repairs, unplanned stops at places that charge admission, or anything else you forgot to consider. If all goes well, you won't need that extra money, and will have a little savings leftover when you start grad school, which sure won't hurt. If all doesn't go well, you won't find yourself with a big debt at a time when you won't have much opportunity to work it off quickly.
I bet people thought I was out of control or something, but I never had credit card debt that lasted longer than a month (the monthly payment). I'm only getting rid of it now because it will be easier to budget when I move out. When I get a job after school, I'm going back to the credit card most likely. I never had problems using it.
