Hey, this question has been asked & discussed a bunch of times before, so I hope you tried a bit of searching.
As far as I can tell from what everyone says, the $/day varies from $5 to $60 for a camping trip - you can really tune it depending on your needs/wants and how hard you're willing to try & how much you're willing to put up with. If you sleep in ditch, never shower and eat ramen and peanut butter it can be very cheap, but if you camp at RV parks with showers and eat in sit-down restaurants it can be much more. Everyone has a different tolerance for that, so "it depends" is kind of the right answer, sorry.
My experience has been that when I pretty much relax about money, camp in pay campgrounds, eat in restaurants about once every other day, I'm spending about $40/day. That's with a stove & cooking some dinners. Pay campgrounds are $6-30 dollars. If you're trying to minimize spending AND want to eat hot food, bringing a stove is the way to go, hot dinners are the most expensive meal you have to buy if you eat in restaurants. But again, you have to decide based on your time/spending/eating preferences.
You *can* camp legally most of the time, not all of the time. This can include pay campgrounds, town parks (often free and legal, but you have to check as you go), legal dispersed camping on forest service or BLM land, asking for a place to stay at a church or in someone's yard/land.
The word "stealth" camping, IMO, does have a certain element of "not getting caught b/c it's not really allowed" - but you can wild/dispersed/free camp without it being illegal, and still want to be low-profile (to not get messed with). But some people do extend that to trespassing, which I don't do or endorse myself.
The adventure cycling maps have a lot of information about free (& other) camping, so if you're a planner rather than a wing-it-er, and you want to save money on the road, the maps can be a good investment to help you find cheap lodging.
Basically, i'd suggest thinking through your own preferences carefully, and be honest with yourself about what you want/need, start a spreadsheet and do your own math. Then add 20%.
