You're laboring under the illusion that I'm opposed to the $300 price tag on its face. No. I'm rejecting the idea that saying yes to be in a bridal party means you're happily agreeing to go along with a string of obligations of time and money because you should have assumed them going in. To a certain extent that is true, absolutely. But IMO getting a ways down the road and saying, oh, this wasn't how I saw things going with X doesn't make you unfit to be in the bridal party. Obviously you don't go in claiming, well I assumed the dress would be $50 max.
Let's face it, there are wedding celebrations that include a lot of things that individually are well meant, reasonable things on their face, yet cumulatively amount to something like death by inches. You're basing your comments on participating in weddings where you are not only close with the bride, but the bridal parties also are well known to each other. Lot's of perfectly great bridal parties are made up of groups that don't necessarily collectively know each other all that well and in those kinds of situations it's easy for people to look at something and say, this isn't a lot of time, this isn't a lot of money, this is what we did with all of the cousins' weddings or all of our friend group weddings, I don't know why in the heck her college roommate has such an issue with it if she's such a good friend. Maybe the college roommate and the bride herself are also participating in another two weddings within a 14 month period with their own hurdles of time and money to jump through. It happens. Your answer may be, well, don't be in all those weddings then. Kind of hard to just write off participating in that way with one or two friends or cousins and then telling the rest, too expensive, when no one thing or one person is picking extravagant or time prohibitive things.
IMO nothing wrong with OP celebrating the evening with the dinner and begging off for the rest.