The bridesmaids have been the unreasonable ones here. They should not have booked anything without a) telling you what they had in mind first and b) checking everyone could still do it. As it is, they ought to be the ones liable for the extra costs as they made a foolish decision without consulting anyone else.
What exactly is it that they've booked? If it's accommodation, why not cancel the unneeded rooms, and at least then they would only be down the deposit? [buconfused] Unless they've booked something like Travelodge-style cheap deal no refund rooms. What else have they booked that can't be refunded? It's really very misguided of them.
I would say you will go at the price originally quoted but you don't think it's fair to have to pay an extra £100 for a) something you won't actually get any benefit from (if it's wasted rooms, for instance) and b) where it's not you that has pulled out. They can't actually make you pay, so make it clear that you will not be paying the extra costs. You will either pay £150 as originally asked, or you will also be pulling out and therefore they will have to account for your share as well.
In fact, can you contact the other remaining hens and suggest you all say the same thing? That way, none of you who have stuck with it lose out, as if you all stick together there's no way this can be forced on you.
If I were the bridesmaids, I would be chasing the drop-outs for money, and/or trying my hardest to either get a full/partial refund after all, or reselling whatever I had paid for to other people.