It's more than I would want to ask someone especially with the other expenses for the wedding...flight, most likely room (if any of them have s.os they won't be sharing with you - tho. it is definitely nice of you to offer)bacholerette party, bridal shower gift, wedding gift, shoes, hair, nails, makeup...Their your best friends and they'll most likely do whatever you want without complaint (to your face), but I personally think you could find something just as nice for less money.
On the other hand, if they're the dresses you have to have maybe you could split the cost with them if its too much for them. if they can afford it all without a problem and you absolutely love them...it's your day and they'll be more than happy to get them for you and you did give them 14 months notice.