When I was in high school, my class ring ended up being a gift from my grandmother - I still have it as a keepsake, but don't remember how much it cost.
My one stepson got a class ring and the other is getting one this year (you have to at least be in 10th grade). I think their mom just paid for them herself and the ones they got were like $400 each. Personally I think that is just crazy but she has always been one to just buy them what they want instead of at least trying to meet them halfway (they do save up for other things though). My daughter is only 3 so I don't have to think about it too much yet. But what makes the most sense to me is to be willing to chip in whatever amount you think is reasonable ($100, $200, etc.) and then have them pay for the rest out of their savings/job earnings - the more expensive of a ring they want, the more they have to contribute.
On a side note, I can't believe all the different design options they have for class rings now. When we got ours 20 years ago, they came in one color scheme (our school colors were green and white, so it was a silver ring with a green stone) and there was 1 guy's ring and 1 girl's ring. And you could have your name, your graduation year, and the designs on the sides were limited to extracurricular activities in school (football, marching band, etc.) Now you can get all these different metals, stone colors, settings, etc. and the designs on the sides include Confederate battle flags, deer hunting, Yin-Yang symbols, skateboarding, you name it.