I think it would be rude to not respond at all. She's trying to fit into your family. Regardless of the way your father talks about her to you, they are dating and have been for sometime. It's not like she's the flavor of the month and wants to drop $200 on your kid. And her buying you a present doesn't obligate you to her if your dad breaks up with her.
Some people enjoy buying presents for others. I think you could kindly state that $200 is too much and that you really appreciate the thought. But let her get you what she wants to get you.
FWIW, DH grandma was set on getting us a "large ticket item" - either a crib, car seat or stroller. We have already gotten a crib and car seat and are planning on a smaller stoller - only $90. She thought that was too cheap and offered to buy a different one! I just laughed (nicely) and said that's not necessary but thank you so much.