Daily cost can vary pretty widely depending on your choices, your route choices, and your camping preferences. Food, water, and camping are the biggest items that you need to have, but remember that in food you need to include a lot of snacks as you will be burning a lot of calories. Also remember that you will likely be wanting a lot of cold beverages. Those do add up. Still it is possible to eat fairly well on $10-15 per day (some manage on less and some spend much more).

On routes like the Trans America, the Northern Tier, or the Southern Tier it is especially easy to keep camping costs low. There are tons of places to stay for free so it wasn't hard to keep our average under $5 or so per day for camping. That was in 2007 and there were three of us sharing expenses so you might go higher, but I think that is a ball park figure if you are kind of a cheapskate like we were on that trip. That meant a lot of camping for free in town parks, some stays with hosts, and some sleeping in churches. and very few stays in more expensive campgrounds. We managed to do that with no need for stealth on that trip. Using an AC route helps some with that, but you can do similarly with a bit more effort with your own route.

Since then I have been more inclined to splurge on a room once in a while and that jacks the daily cost up quite a bit.