I think if you make it clear to them when asking them to be part of the bridal party then it's fair. Also, you could ask them to pay for it as their wedding present to you.
From what I've heard, most bridesmaids never wear their dresses again, so them getting to keep it doesn't really make a difference.
Plus, as I've only just found out, there seems to be all these expectations about bridesmaids helping out with planning the wedding, but little expectation for the groomsmen to contribute in that way either.