Schools that meet full need do count travel and books/supplies (arbitrary estimate) into the full cost. The summer expense (camps, unpaid internships etc) and optional school activities are considered discretionary I think so they are not part of the full cost. Your EFC should be considered relative to full cost and not just tuition plus room and board.
@Serioussweetie Here’s an old thread I found which outlines some of the costs away from tuition. Some of the stuff listed, in my opinion, would be a cost even if your child stayed home, so not specific to boarding school. Then, there are costs you save, most specifically on all the food your teen won’t be eating at home. 
We were more local to our kids’ school so our costs were much cheaper than the costs @ChoatieMom referenced as we did not have flights to worry about nor hotel costs. We were also one of those mean parents that didn’t sign our kids up for the laundry service. Books ranged about $500-1000 the first year, less the subsequent years as my kids were good about buying many books from friends much cheaper than the bookstore price. Never had to pay for uniforms but did spend about $100 or so on “team wear” but my guess is that would have happened at the LPS as well.
Absolutely. This is a very good question to ask, not impolite at all. Very sensible.
Don’t forget the expense of a formal wardrobe (and dry cleaning for woolens) if you do not already own suitable clothing. There will also be some potential pressure in cases where your kid get invited on trips with friends… or wants to buy team/dorm spiritwear.
My kid’s school (Putney) gave us an estimate ($1,200) of added school expenses to consider but we have been able to keep that expense much lower. We are a few hours drive from campus, no dress code, found used books online at a major discount as well as kid barters with older students for old books. Kid has minor (used) sports equipment expenses. Coming to visit campus overnight twice a year is a bigger expense with eating and lodging but that can be kept on a tight budget with a little creativity…including…camping out in scenic Vermont 
Why do you have to decline a financial aid offer. We are receiving postcards from colleges telling us that accepting/declining financial aid is an online process. If you don’t put the deposit down by May 1st - wouldn’t the school know you are not coming thus you are declining the offer?
@Cintimom1: This is the Boarding/Prep Schoo subform
Cintimom, we’re talking about prep schools…somewhat of a different process.
@Cintimom1 this is the prep school forum. The question was re: boarding school.