Hiking through the Appalachian Trail is actually free and if you want to just go hiking through the entire Appalachian Trail you can do so without spending any money.
But the Appalachian Trail does pass through some places and state parks that will require a fee for you to stay there overnight.
But you can sometimes camp in areas in a tent for free.
If you do want to spend time in state parks etc while on your journey through the entire Appalachian Trail it's recommended to have at least $1,500.00 per month on you.
The average spending per person hiking the Appalachian Trail is between $5,000.00 to $7,000.00
Hiking the entire Appalachian Trail is also a very difficult thing to do and some only hike halfway and then return.