If I was doing a wedding at home, I probably wouldn't be paying for the dresses (I've paid for my BM dresses in every wedding I've been in - which is a lot haha). But as we're asking them to travel, we are paying for their dresses and 3 nights of their stay. However long they want to extend their vaca beyond 3 nights is on them and they're paying for their flights, so I would think the flight alone is more than what most would spend on the dress in a 'normal' wedding. Granted, hopefully they'll get a fun vacation out of it as well but...we wanted a big portion of our budget to splurge on taking care of our wedding party; as a way of saying 'thank you' for loving & supporting us over the years.
But every DW is different, so I don't think there's a right or wrong answer.