I agree with your line of thinking Doc.
Gear: Equipment I replaced during my thru were a stove, sleeping bag, broken hiking poles, shoes (4 additional pr.), socks and other clothing. Total over $900.
Weather: On a few occasions I went into town during bad weather to dry out. Spent a few days at Shaw's in Monson, ME during Hurricane Irene. Depending on where you stay that could cost $100 a day including eating at a restaurant and other town expenses.
Injuries: Had to go home from NH for an ankle injury. Two weeks off the trail and $200 in travel expenses.
As you can see it adds up fast. Good to be prepared then to come up short and have to end early.