There really isn't any real way to figure out what your day to day costs are going to be bike touring overall. Costs are almost 100% local. Some places are a lot cheaper than others. $40 a day is a pretty cheap-- small groups are often cheaper than solo trips, per person. Urban stops cost more as a rule.
Local bike clubs often do sagged tours for around this cost ($40-$60 a day)--- they are often a really good deal to get started in touring.
Of course, it's very possible to go cheaper....way cheaper. I did when I was younger, but I was really poor at the time. But my question is...why would want to? (Unless you're super broke).
I have a friend who rode coast to coast here in the US, spending some crazy small amount like $12 a day. Stealth camped the whole way, ate chick peas and other bulk health food bought at big urban centers, did 100 miles a bunch of days. He managed to ride clear across North Dakota with almost no human contact, partly because he didn't have any money to spend and partly because he was sure those Right wing, gun lovin' No Daks would hate his Vegan hippie lifestyle. (I've been to N.D. a bunch of times and think its full of really nice people who love cyclists, but I couldn't change his mind)
Spending money locally buys you contact. You meet people. It helps the local economy, it makes people have a positive view of cyclists. I understand that every cyclist has a budget, and some are a lot bigger than others. As a teen, I started touring with 2-3 day outings, fueled by 3 cans of pork-and-beans and a jar of peanut butter. I'd buy day old bread and orange Shasta at little stores along the way.
I spend a lot more now, but I can afford it.
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