I've had really good luck buying from http://www.2filter.com/prices/specials.html.
He's very knowledgeable and good to talk to on the phone and will advise you as to the best-but not necessarily most expensive -filter for your needs.
Well, just as a chain is only as strong as its weakest link, a lens is only as good as its worst piece of glass. Multi coating help prevent flare, which can be a big issue when shooting with filters. I wouldn't recommend a filter that wasn't multi-coated, but I'm sure in most situations you would be OK without it. Here's a Hoya multi-coated polarizer for $85.00: http://www.bhphotovideo.com/c/product/116781-REG/Hoya_016676_67mm_Circular_Polarizer_HMC_.html
I only have experience with B+W, but people say good things about Hoya as well.
It costs significantly more, but Singh-Ray has a great "lighter, brighter" polarizer - it's supposed to be just as effective in terms of polarization, but doesn't cost you as much light (polarizers tend to eat about a stop in terms of exposure).