I started the Northern Tier spending very little by cooking food from grocery stores on my alcohol stove. However, I wasn't eating enough and I was loosing weight (oddly, I didn't feel hungry). I started eating out for one meal per day, then two and, eventually, I ate out almost all of the time. Not surprisingly, food quickly became my biggest expense, and I bet I was spending about $30 per day. Note: I still lost weight, even though I ate, I bet, close to 6,000 calories per day. (I averaged 83 miles per day for the 56 days I took.)

Of course one could cook that much food along the way, but that is a talent and interest I don't have.

Camping was much cheaper: I was quite pleasantly surprised by how many towns allowed free camping in city parks. But, I still bet I averaged close to $10 per day. BTW, I used Warm Showers three times and I got a hotel room once.

I had very few mechanical problems, so not many expenses here: two flats that I patched, but I did replace my tires, preventatively, at ~3,000 miles. I also replaced my pedals because one was making a clicking sound, but Shimano ultimately covered those under warranty.

I didn't have too many other expenses, and I'd bet my on-tour cost averaged something between $40 - 50 per day, and I felt quite comfortable.

As others have pointed out, traveling with someone drops your costs considerably in terms of sharing campgrounds and, IMHO, most dramatically, by making it easier to cook more of your meals.

It is very nice to have a credit card handy, just in case....