I think your friend should have some say in the shower plans if she is being asked to contribute $200 (btw, how many other ladies are to contribute?), and if the renting of the location costs this much, I can only imagine how much the wedding is costing!!!
If it had previously been agreed upon that mom's making the dresses means attendants would "pay for the dress" by contributing $ to a different part of the celebration, then so be it.
However, ime, when dresses are made instead of bought it's always been to save money, not to finance a shower. Seems kinda odd that the bride and her mom are planning the shower, too.
If your friend can afford (and is willing) to pay for the shower, then I'd just let it go. If she can't afford it (or doesn't want to), then I think the most polite thing to do is respond immediately and say something like "I am sorry, I cannot afford to contribute $200 to Lxxx's shower, please let me know what else I can help with" or "...but here's what I can contribute" and then send that amount.
As an attendant, I think there is an expectation (and responsibility) to help, but if the occasion calls for spending that much money, it should not be financed in this way, imo. tacky tacky tacky
Rachel, married to Andy, mama to Aubrey, Lauren, Sasha, and *surprise* Baby #4, due November, 2011