It depends on your destination. For example, some parts of the country, including Hawaii, have off-season airfare costs that sometimes include a surprising discount. Sometimes, too, airlines such as SWA offer the $99 one-way travel, but this may not be a savings if you're not going to the city being advertised.
A road trip can cost more because of the cost of fuel. If you take a road trip, the value are the stops along the way. For example, if you plan a summer trip to the west, you could camp and visit Durango, Colorado, the Grand Canyon, Sedona, Arizona, and even make the stretch to San Diego.
The cost is entirely up to you. Set a budget and then make an outline of your options including approximate figures for fuel, if driving, or airfare costs. Then figure hotel costs and camping cost options. If you have children, then you need to figure in any cost for entertainment, but fun activities such as hiking are relatively free of cost.
Plus, when you drive somewhere, you save the cost associated with car rental. There are also some KOA campgrounds that now have some awesome all inclusive cabins for families.
Lastly, check out vacation packages because sometimes those save you money. Of course, you have the websites that help find the cheapest rates on airfare and hotel.
You need to make a plan first, then outline your options to compare costs.