Experience helps, too. After time (or with the help of others), you can better ID things that you will want to budget for.
The Magical Express situation provides a great example of how having spreadsheets for each vacation plan can help you compare total overall cost. Otherwise, you might figure the cost of a rental car as a given for all parks, when you might not need it for an on-site Disney visit.
We've never created a budget for our trip. Generally we pay for our hotel, tickets and rental car months in advance. We take quite a bit of money with us for food, souvenirs and anything else. The way we've always done it is to pay for all of our big ticket items prior to leaving, then when we go on vacation, we try to take anywhere from $1,500-$2,000 with us. We always bring a lot of money back home with us, so we toss it into our "vacation fund" for the next year.
This is something I definitely want to look at though, perhaps it can help us even more! Thanks for putting this together.