interestingly, i do think that there is a difference in quality.
I'm using cokin filters now, and i find cokin's filters a bit... ermm .. sad. I was previously using tiffen's polariser, and so far, I can't replicate the shots done using my tiffens, compared to the cokins. Of course, most pro's that i read about would be using singh-ray polarisers instead of the cheap cokin one.
Another issue that would affect the price would be the colour cast. Most people (like me) would just ignore this, but I'm told that certain brands colour casts, although I have no idea which.... its one of those i heard from a friend who heard from his friend thing. (added: ok. if you google, you'll find its a fact. for people who shoot slides, this colour cast destroys a lot of shots. Most polarisers do have a bluish colour cast. Thats why there's such a thing called a warming polariser. (singh-ray!) Some have greenish casts as well ! . arrgghh)
[later added ... cos i just remembered]
I think another point would be how much light loss you get from the polarisers as well. The better ones generally lose about 1 1/3 stops while others lose about 2-2 1/2 stops.