A lot of schools have a standard, fixed tuition amount that will apply to a set amount of hours. So, tuition might be $25k for up to 18 hours, let's say. If you take 12 hours, it's $25k. If you take 15 or 18 hours, it's still $25k. If the school you're looking at has a fixed rate like that, and if you can fit all your double major courses in to that many total hours per semester (18 in this example), then double majoring won't cost you any extra at all. If you can't fit all your double major courses into the credit hour limit, then simply multiply the hours over and above the 18 hour limit (or whatever your school's limit is) to get a rough estimate of the cost of the second major.
If the school you're looking at charges tuition by the credit hour, plus fees, then it'll still just depend on how many more credit hours you have to take, over and above your first major, to fulfill your double major requirements. As somebody else mentioned, you can often count one class towards both majors (i.e. no additional cost for that type of class under the "cost per credit hour" system). If you can't count a certain class towards both majors, simply multiply the cost per credit hour times the number of extra credit hours for your second major to determine a rough estimate of the cost of your double major.