I dont typically tent camp, but I have seen the rates at the various campground we visit.
On average in Alabama, the tent sites are $12-$16 per night, of course depending on the campground, whether its lake front or not, etc...
There are many campgrounds that have an RV campsite, as well as a tent pad too. Some dont charge for the extra tent there, and some do, usually a smaller fee than the above price. That works out good if you have a tent camper and and a camper all going together.