Funny i just got done researching this.
I am on a budget but want pretty good quality, not junk.
You can create your own variable ND by stacking 2 polarizers. Then you have both ND and polarizer for one price.
[url=http://www.dimagemaker.com/2008/02/23/using-crossed-polarizers-for-a-variable-nd-filter/]http://www.dimagemaker.com/2008/02/23/using-crossed-polarizers-for-a-variable-nd-filter/[/url]
According to tests, best polarizer for the money is the Marumi DHG Super line. 2 of these is about $160 on AMAZON. But that covers [u]both[/u] your ND and polarizing needs. Great deal and tests show this is as good as much more expensive filters. Will be thicker than a single filter so might need to get bigger filters to prevent vignetting with extreme wide angle lens.
If you dont want to DIY, according to tests, best buy in a [u]reasonably[/u] priced variable ND is this one ($125):
[url=http://specialcameraservice.com/49-fader-nd-mark-ii]http://specialcameraservice.com/49-fader-nd-mark-ii[/url]
But then you will also need to buy additional polarizer.
I'm DIY ;D