I think you have to base the answer to your question on what you know about your grandmother. Is she living solely on social security and $200 is a lot for her? Or do you think she’s just not aware that the wedding dress you want costs more?
Grandma was probably married 50+ years ago and may not have gone wedding dress shopping in a long time, and therefore not aware what prices are. Why don’t you invite grandma to go shopping with you and let her see the style of dress you want and the price tag that goes with it.
If she can afford it maybe she’ll give you more money towards the dress, but if she can’t, thank her graciously and pay the balance without telling her.
Above all, make sure grandma knows that you appreciate her gift.