Just a word of caution about buying a cheap one and/or just sticking two CPLs together back-to-back (or is it front-to-front?). At very-low and very-high filter factors, you can get a colour cast, I've tried building my own and managed to get it at a point of the whole shot going rather green or rather purple. Even with digital that's useless, I'd hate to try to remove a colour cast from film. Also, you can get a cross-hatching effect in certain circumstances.
I'm all for saving money on a lot of things (and very often do), but with vari-NDs more than anything you really do get what you pay for. I've never heard a bad word said about Singh-Ray or Heliopans (except the price, see here for a simple comparison, Singh-Rays and Heliopans are 10x the price of the cheapest Vivitar/Polaroids).