he's stating the total cost per family, not the increase from the current cost, so you have to use $7 and not $2.50.

$112 for a family of four on a round trip. That's $28 per person. With 4 segments on a r/t that's $7 per segment.

So the error in his statement is in the word "additional", he should have said "total".

edit to add: steex is saying the same thing, but faster than me by a minute.