We aren't too bad with hidden costs yet. Just things I didn't think about as part of a wedding (such as marriage before, rings, dog boarding etc.). Obvious costs, but I was thinking day of only.
The resort we are getting married at does have some hidden costs, but all you have to do is ask the right questions.
For instance - for the ceremony, every guest over 25 people is $14 (so the chair set up, the champagne toast and cake). For the cocktail hour it is $12 per person over 25 and the dinner is $40 pp over 25. So every (adult) over 25 is going to be $66. I budgeted for the full guest list 'just in case'.
Our resort will charge us for decor we bring, but just make sure you keep your WC in the loop as to what you want to bring down and they can give you estimates. For instance, the resort will charge 25% of their fee to set up our items. However, I am doing chair sashes, just on the back and aisle chairs. They charge $1 per sash. BUT I am also wanting some tissue poms tied onto the aisle sashes, and she said they would include that in the $1.
Same for centrepieces - show them photos of what you want to bring and they'll quote you the price. That way, when you get there you have the amounts documented so you don't wind up getting a different number there. I am keeping it simple - first choice is some nice driftwood for the centre, and some pillar candles. Maybe some jewels or flower petals on the table.
I assume EVERYTHING will cost me more, so I'm constantly asking questions and re-working our numbers.