I keep getting folk into high dudgeon on this chatroom when I query the value of buying "quality" filters rather than cheap ones. I say that as the owner of expensive Lee filters (the ones for my Nikkor 14-24mm lens are VERY expensive) and a range of "Chinese Cheapies" some of which cost less than £5 on eBay - and I can say that, for all practical photographic purposes (by which I mean producing A3+ salon-quality prints from a D800/e Raw file), it is absolutely impossibly to detect any difference in image quality with the naked eye.
Of course, one often gets hit by the inane cliché, "Why put a cheap filter in front of an expensive lens?" The answer is that it doesn't make the slightest difference.
As far as variable ND are concerned, I use an "own brand" one from the excellent "7-Day Shop" which, about 3 years ago, cost around £35 for the 77mm version. I believe that the price has now dropped to around £19.
Bearing in mind the obvious defects in any variable ND filter (as far as I know, they all are essentially two polarising elements contra-rotated relative to each-other), I suspect that it is as good as a more expensive one although, to be frank, I don't use it very much as I prefer to use fixed-value ND filters, either Lee or Chinese Cheapies depending upon which lens attachment I am using. I tend to think that two, or even three, standard NDs stacked to obtain the desired total value, will give a better result than a variable ND (although maybe not as convenient).