Um, if you want cheap and already have a mm circular polarizer, pick up a linear polarizer
(don't know if you can find 'em that big secondhand) and screw them together with the linear one on the outside. You might have vignetting issues you don't like if you shoot wide and all variable ND setups have color issues.
If you're really serious you might be better off going with a set of neutral density filters.
The original Lightcraft was very poor. I returned mine. I would say a good variable ND is expensive. Getting rid of the color cast at high ND is the trick. You also may find thin is better, for wide angle lenses.
I have a Hoya variable ND and I like it a lot. It is sharp, thin profiled, and doesn't have much color cast. It's also quite a bit less expensive than the singh ray or Heliopan.
It's the best vari-ND i've tested. However you should be aware that variable NDs and wide angle lenses don't mix well. Variable NDs create X patterns near the end of the their darkest range, which show up clearly on wide lenses. The problem goes away around 50mm (on full frame), but at 24mm or wider you won't be able to take advantage of the full range of the filter.
although I have a few variable NDs of different brands, I find that I am going more to set ND filters because of the X-issue with wide angle lenses and the color casts.
