Something to sharpen a snowboard/skis?

I want to be able to use it on both. I've seen stones and metal things that are 90* angles. I really just want to get the little bits of light rust off at the beginning of each season. Which is the best one? I don't want to spend a lot (possibly under $15...I'm not sure how much these things cost though). Can the same thing be used on my skis and snowboard?