Like everything else in the world, our upcoming road trip can be boiled down to two things: time and money. It's the eternal trade-off. If you're making money, you've rarely got the time to spend it. If you've got the time, you've rarely got the money to spend. Hence our plan to make money, then make some time to spend it.
More to the point, what any expedition needs is a realistic budget. In our case, this is just our personal stash, but maybe you're one of the ambitious or sponsored types who gets grants or expenses paid or something like that. (Of course, you probably don't need to be reading this blog about budgeting for expeditions if you've already figured out how to get other people to give you money...)
So, what are the budget essentials for any good road trip?
You can't live without it. If you're planning on cooking all of your own meals, you can typically get by on $5/person/day, give or take. This assumes that you 1) buy in large-ish quantities 2) put some thought and planning into your food purchases. This also does not take into account "luxuries" like beer, dessert, or "outrageous" expenses like cheap Mexican food.
If you are taking off on the great American road trip, without a doubt, one of your largest costs (if not the largest cost) will be gasoline for whatever automotive transport you decide to take. I suppose you could walk, bike, hitch-hike, or hop a freight train, but barring those modes of transportation, your means of locomotion will cost money.
The biggest consideration here is the number of miles you will travel, the fuel efficiency of your vehicle, and the price of gas where you're going. This last one is important if you're some place where the cost of living is relatively low (ie. Alabama) and going somewhere the cost of living is a bit higher (ie. everywhere else). Also bear in mind that while Google may tell you that there are 782 miles between your starting point and your destination, Google did not account for the 20 mile one-way fresh-veggies-and-beer run you made from your camp in the middle of the woods to town, nor did it consider your unplanned 15 mile drive to the nearest pharmacy for more ibuprofen. In other words, give yourself some buffer here lest you nickel and dime yourself into being too broke to go home.
This one will depend on the length of your road trip and how frequently your insurance bill needs to be paid, but rest assured you'll want to have some for your health and your car at a minimum. (You might also consider traveler's insurance, accident insurance, rescue insurance, and a host of other options. The American Alpine Club offers a variety of insurance plans for its members). Of course, having insurance doesn't exactly fit the "dirtbag" ideal of many American climbers, but if you've ever ended up hospitalized on a climbing trip, you can appreciate the value of insurance.
Depending on the length of the trip, it may also be advisable to set up a new bank account or two for expedition-specific funds. This usually doesn't come at any cost and is easily done at the bank or even online. Having a separate account to draw from makes it easy to track your spending on the trip and actually follow the proposed budget instead of going to town every night because you're too tired to cook in camp. This also makes it easier to keep your "real life" funds separate from your "road life." All of your regular auto-drafts (insurance, rent, car payment, that sort of thing) can come out of the usual place and your trip funds remain intact. You did set up automatic online bill pay for all of your regular expenses before leaving, right? Taking the time to get 3G or wireless signal on occasion can also save headaches from unforeseen financial snafus.
Don't forget about things like lodging if you want to get away from the tent on occasion. Also, consider things like showers, road tolls, Starbucks, gelato you couldn't pass up, emergency gear repair, and anything else that might sap your funds. It's not a bad idea to have just a little cash in reserve.
Once you've got all these needs covered, there shouldn't be much spending of money while you're actually out on the expedition. As my co-workers at the North Carolina Outward Bound School say, it's hard to spend money while you're living in the woods.