All the "other stuff" related to traveling you need to spend money on. The trip itself (flights, hotel, meals, activities, etc.) seems to be the cheapest part of the total cost sometimes!
Any place I go, I need to either pay for parking or a town car to get me to O'Hare. Town car is at least $60 plus tip one way, parking in the economy lot is something like $105 for 7 days. So $105 to $150 depending on if I want the hassle of taking the tram from/to E lot.
Once I get to where I'm going, I need transportation to the hotel. With WDW that's not an issue due to ME, but other places (like DLR) that's going to run me upwards of $100 (for town car, less if I can do something like Super Shuttle).
Then, while I don't have a pet right now, we plan to get one soon so that's kennel fees of around $150 for a week, although sometimes I can have a friend house sit for us. That's not a given though.
Then there is that week before the trip, when you start to get all the stuff you need to pack collected. Oh my bathing suit doesn't fit, need to buy a new one. Don't have enough sunscreen, need to buy more. Can't find my hat to keep the sun off my face, need to get yet another one (and then find my stash of old hats a month after I return from the trip). Need new travel sized shampoo containers (DH trashed the last ones, or can't find them...probably stashed with my missing hats...), snacks for the plane, new sandles because my old ones are just about shot....on and on it goes until I drop what seems to amount to a car payment - for a very nice car - on just trying to get us out the door!
Then we actually get to the airport...DH insists on checking bags, $50 round-trip (I refuse to check more than one). Then DH wants water once we're through security. $5. I want coffee because I've been up since 3 AM trying to get last minute stuff done, $5 at Starbucks. I realized about half way to the airport I forgot my sunglasses. $20 at the sundries shop.
DH decides he's hungry. No he doesn't want the snacks I brought, no he doesn't want to wait to buy a snack box on the flight. So $35 at Chili's Too at O'Hare's B councorse (because DH also decides a beer or two at 10 AM will make it easier to sleep on the plane...). At least the uneaten snacks I have will keep DH out of the mini bar once we get the the hotel....
When people say their trip cost them $1200... I can never figure out how. I seem to spend that much before I even land at my destination!
So what are your "hidden" travel costs?