Dilemma: You're asked to pay for a bridesmaids dress you can't afford, and everyone else seems fine with the price, so you don't want to rock the boat.
When the bride shows you a few dress "finalists" to choose from, ask if you can speak with her privately. Or, if she sends the photos via email, respond to the sender only; don't hit "reply all."
What to say: "These dresses are all very pretty, but they're more expensive than what I can afford. I want to be sure I have enough funds to help throw a fantastic bridal shower for you as well. Can we look for a few equally-pretty options that cost around (insert $X amount here)? Sorry to be a financial bummer—I'd be happy to find a few more options in the same color palette and style, if that's OK with you."
By offering to help the bride find a solution, you may get a far better response than stating all of the reasons why you can't afford the one she originally picked out. Don't be afraid to speak up: You may regret it if you don't, especially if you find out afterwards that all of the other bridesmaids thought the price was outrageous, too, but no one was brave enough to say anything!
If you hear back from the bride that your new dress options are welcome, get it done right away so that the bride doesn't get anxious about this task taking too long.