Every private campground is different how they handle drop-in's vs reservations. State sponsored campgrounds (like State Parks or Federal Forests) are found on the state DNR (Department of Natural Resources) web sites. Federal campgrounds, here's a start: http://www.recreation.gov/
If looking for overnights,in addition to Wall Mart and Cracker Barrel, don't forget about private campgrounds or campsites maintained by private clubs, like American Legion, Elks, Masonic Lodge, Moose Lodge or even military. If you are a member of one of these organizations, consider contacting them and see what's available. Also, if you are the member of a specific religious group, church, synagogue, mosque, it's possible you might be able to get an overnight in the parking lot if you contact the right spiritual "grand poo-bah!" (We belong to a church and we link with other ministers and churches and inquire about overnighting in their church parking lot. Sometimes we can, sometimes we cannot, but it's worth a shot if you belong to something like this).
There again, to answer your question, there's no standard. Every campground is different, and prices are different for every campground too.