I really think this is more of a personal choice. If you are willing to buy them something expensive, then do it. If not, then don't. I mean they are your parents, they love any give you give them!! You also may want to consider making something for them.
I think that you are the only one who could decide what you want to buy for your parents. Maybe you could figure out what your parents want. Is there something they really really want? If not, buy them something that is around 20 to 30, i guess. Or you could make something.
To be honest I never get anyone in my family anything for Christmas except the kids. Usually I give them $20 each. With my family I often visit and bring 2 or 3 bottles of different wines. We have a big family. From experience I can tell you that you should NEVER give jewelry or clothing. Everyone has different tastes and it doesn't matter how cool you think it might be, the receiver might disagree although not to your face.