When I was MOH for one of my friends I assumed I would be buying my own dress, so they may assume the same thing. I would have a sit down with each one though and tell them your expectations for their part in your wedding, including parties, speaches, dress shopping, and dress buying. If they really have a huge problem with it then let them know you may have to re think their roll. At the same time though keep in mind how expensive it can be to be in the wedding party, so try to take it easy on their budgets too, if you are having a modest wedding you maybe able to find dresses you like at Macy's or a Ross dress for less. If you look you will find something that is pretty with a price that you all can agree on.