Please recommend a viewfinder of two categories: expensive but the best bang for the buck, and a dirt cheap(under $20) viewfinder. If you have any experience with the dirt cheap viewfinders please tell me if the product was okay.
If by viewfinders you mean something like the Zucato Pro or the Hoodman, I own both and they are priced accordingly. The Hoodman is almost worse than worthless, the Zucato Pro (expensive) but worth every penny.
For $20 you might as well make your own hood for the built -in viewfinder, because any kind of optics aren't really going to help you focus at that price.
For shooting video I would recommend an electronic viewfinder (EVF), definitely not dirty cheap.
I have a SmallHD DP4 which I consider the best bang for the buck. Zacuto also makes good EVF's
I've heard very good things about TV Logic's EVF but never used one myself.