I've ridden x-USA a half dozen times and toured 100,000 miles - so I have some experience.
On my first trip in 1987, I budgeted $20 per day for 70 days, plus $100 for a total of $1500.
There's been, at least, 100% inflation since then - esp. in campsite fees and small portion grocery items.
Not to mention that you learn how to save with experience.
The biggest avoidable cost is alcohol - a couple of microbrews will set you back $10 to $12 -
And if that gets to be a daily habit, then your budget will quickly be trashed.
The biggest controllable cost will be camping/lodging.
If you follow an ACA route, esp. the TransAm, there will be more low-budget options.
Unfortunately, most state parks and national forest campsites run $15 to $20.
Private campsite can run $30 to $40 - killer if you are solo.
Oregon, Wash., & Calif have hiker/biker campsite as do western national parks.
These run $5 to $8 per person - still not cheap.
You can camp for free on all federal USFS and BLM lands in the West -
Provided you know where they are and follow fire/sanitation regulations.
Also, tiny Great Plains towns will often let you camp for free in their town parks.
The bigger the town is, the less likely. But Nebraska and South Dakota are quite good.
Of course, there are host websites like Warmshowers and Couchsurfing -
Then there is also the options of contacting churches in small towns.
Catholic churches usually have the rectory where the priest lives right next door.
In small towns, Protestant ministers usually live close by.
Choose a church that has a nice grassy area in back and some privacy.
Quite often, they will let you cook and clean up in the rec hall.
There will be times - after 3 days of rain - where you will just have to get a motel.
In fact, if you are wet and exhausted, it really makes sense to do so.
The risk of a serious accident goes up geometrically when you are totally wiped out.
It is money well spent.
There are other options out there.
But I'd say $20 per day is frugal, $30 is moderate - esp. in the East.
Happy trails! - J