Noise canceling ear buds are likely best. Regular ear buds (ie: the ones that come with ipods) don't seal the ear canal, so a lot of outside sound gets in - but noise canceling ear buds will help in that regard. The difference is that they look like little plugs that go in the ear, not just 2 mini speakers. Usually they will come with 2 - 3 different sizes of outer rubber (the plug) to fit different sized ears. Places like Best Buy will have quite an assortment of these ranging in price.
I'm not sure what is available (and recommended) in a $30 range, but maybe someone will have a suggestion or two. I think this is a prime example of "you get what you pay for", however sometimes their are hidden gems that only a few know about in the lower price ranges.