Location as was said plays a big part in this. In some places there just isn't much to spend money on. What are you going to spend money on in Mitchell OR, Twin Bridges MT, or Pippa Passes KY? Touristy places like the Pacific Coast or larger towns offer more chance for spending money.

What the focus of the trip is is a big factor too. If the main focus is the riding and meeting the local folks, I think it keeps the costs down. If you are doing lots of off bike stuff you have more chance to spend money.

I can imagine myself doing a sub $15 per day tour if I really wanted to and I can also imagine maybe approaching $100 per day in a different circumstance. I really have a hard time imagining myself spending much more that that unless I really change my style of touring to something entirely different. Still, something in the $20 to $40 per day range most likely even if I don't make any particular effort to be frugal.

My tastes run toward simple relatively inexpensive choices, but not depriving my self either. Mostly camping with a mix of cooking and simple restaurant meals is the style I like. My guess is that most riders would average $20-40 per day if going that style in the rural US. I am not poor so I don't need to be more frugal than that. OTOH if I were rich, I am not sure I would spend any more while on tour unless I chose to tour in a different location.