Does it cost more to have a double major?

When you matriculate, you are no longer paying for classes by the credit. So, it is cost-effective to take as many classes as you can reasonably handle, up to the limit your school allows. The bean counters at colleges are no dummies, so they put a cap on the number of credits students can take each semester, because they don't want to encourage too many kids to graduate in less than four years. Frankly, there's not much difference between earning the second major and earning a minor, so you should load up on the courses as much as you think you can handle to get the double.