I have been reading on several sites that the cost of hiking the AT can be estimated at $1 per mile - so approximately $2200. With a 6 month hike = roughly $12 a day. This seems absolutely incredibly low to me unless you are not factoring in a lot of drop boxes filled with supplies.
If you think of supplies, laundry, showers, and the weekly pizza & beer not to mention the hostels every so often - I'm thinking at least $3 per miles. Otherwise, you're going to need a heck of a lot of trail magic....
Now if someone wants to eat Ramen the entire trip, it could probably be done but this does seem to be low to me.