It varies too widely to give a good answer. Location and personal preferences both can cause extreme differences in cost.
It is possible to go for $10 a day in at least some locations, but I am on vacation and want to eat well, including some restaurant meals, so I am more likely to spend more. Then there are campsite costs that can range from $0 to $40 or so depending on where you are and where you choose to stay.
My personal approach is to have much more $$$ available than I will use and spend based on what luxuries I feel I want or need at the time. So far this has meant that depending on the trip my costs can vary from $15 to $30 per day.
BTW, if I do as Cyclebum suggested and "subtract the cost of staying at home" some tours might pay me to go rather than cost given the high cost of driving when at home.
Oh and any of the numbers quoted above do not include airfare or other transportation costs for getting to and from my tour location.