General guidelines are going to be all over the place.
We don't know how far you are going to hike a day or how many down days you are going to take. The US length is 3,100 miles.
What is your food tolerance? Are you willing to eat the same meal up to several servings in a row (single serving is significantly more expensive)?
Are you willing to eat spam on the day you do a re-supply? Can you make some of your own trail food?
Are you going to do a stay over at the re-supply where you can gorge or do you want to take a day off on a peak for the scenery?
Are you going to pre-mail to pick ups or buy as you go?
You cannot mail fuel so for that you need a store.
Even cooking your own food cost could easily swing from $10 to $20 a day.
How many restaurant meals do you budget? How many hotel?
If you plan for 12 mile days but end up with 14 mile days you will have about the same calories. Question is did you budget properly for 12 mile days. If you have too much food you can leave it in a free box. If not enough then supplement in a store.
Plan your route for the first month and price it out. Plan your stops. Call into some of the stores and get some prices. Price camping fees.
Does part of the route allow wood fires that you don't need fuel?
You are not going to easily get the detail on the Internet but get some maps and plan out the first month. You could use that as a monthly budget. But you do need to account for the higher you get your resupply stops will be more scarce and more expensive.
The more you plan the better you can control cost.