Your snowflakes are really gorgeous, as is all of your work. 50c pp for cake is highway robbery; who's ever heard of a wedding cake for 50c pp??! My gut reaction is, if she can afford to have 200 people at her wedding, she can certainly afford the rock-bottom price of $200 for a cake, but then again I don't know a darned thing about her and have no right to make assumptions. It's just that, you said she came to you b/c she knows you're doing one in April, not b/c she knows you're an independent baker and she can steal a cake from you. Still she's trying to take advantage of your position as a fellow parishioner.
Regardless, if it's worth it for you as a practice cake, why not get paid for it? At least you could recoup some of your costs. But I totally agree that you must beware underpricing your professional work, as word will travel FAST. If you really want to do it, even at her price, do a contract which clearly outlines the actual price of the cake and then states something about the nature of the discount.
Oh and btw I love the dummies idea if that cuts down on your time and effort (and cost): she gets her price, you get your practice, yet she still is forced to see that you can't get a wedding cake for 50c/p without sacrificing something.
Just my humble opinion. Good luck with making your decision!