Who is responsible for transportatin costs to visit out of state parent?

We have three children and at the time of the divorce we both lived in the same state. My ex-husband moved out of state and now says I have to pay half the tranbsportation costs. The divorce decree states that we split the cost as long as we reside within 50 miles of where we lived at the time of the divorce. Who's right and what is likely to be decided if he should take me court to have this modified.