As much as I wish our road trips were always to exotic or adventurous locations, for the most part they seem to be visits back to where our siblings and parents live, or to visit friends that have been posted across the country. With gas prices increasing, we've shied away from taking a lot of trips for "us," and instead we tie our love for travel into knocking off visits to loved ones. (I know the "knocking off" part sounds unloving, but there's quite a few of them spread across cities, provinces and countries. We have to be deliberate to fit them all in.)
The financial approach to the road trip has varied a lot over the past few years, but we've seemed to develop a system we're both pleased with. Here's the steps we took last trip to keep the budget in check:
1) Apply the food budget from the days we'll be away to the trip.
Of our withdraw of $350 twice/month for food, transportation and incidentals, a good chunk will go to the grocery jar. If we will be away for five days, we will then take out the equivalent of 5 days worth of groceries and put it towards food for the trip. The same goes for gas. If we end up eating out a bit with family, or driving from person to person to person, we'll spend more away than we would at home for those 5 days worth. That extra money will come from our kitty, which is point 2.
2) Use the kitty.
We set aside a portion of each pay to a specific bank account that covers our miscellany. Basically the things that come out of it are clothing, gifts, and smaller-scale trips. One rule of the kitty is that we have a discussion before we dip into it, and we both try to keep it rare. Once it's gone, it's gone, and if the account ever got to 0 we'd be stuck with very little flex until we built it up again. So we try to make sure that doesn't happen.
3) Plan expenditures ahead of time.
The town we live in is teeny-tiny, with very little "shopping" to speak of. So whenever we go away, we generally end up picking up a thing or two we've been putting off for a trip. Those items are discussed, and a tentative number is settled on for how much we will actually be spending. Sometimes, of course, that number will be off, but for us actually sitting down and looking at a schedule of days can give us a pretty good budget idea.
4) Eat in when possible.
The easiest way to lose control of the budget is to eat out a lot. Again, with a small town, things like ethnic foods and high-end restaurants are hard to come by, so it's a big temptation when we're away. We tend to save the trips out for celebrations with friends and family. (This past trip we celebrated our 4 year wedding anniversary at the place we got married. Some things are hard to put a price on.)
Mostly, it comes down to planning, and being honest about numbers. The more you know ahead of time about where you'll be and what you'll get up to, the more firm and realistic a budget you can produce!