If you are looking for a thin one, get the Nikon CPL-II. It cost only ~200 and is as thin as you can get (just as thick as normal UV filters). Not to mention its image quality even exceed or match the B+W MRC's.
Never judge a filter by it's retail price, as it has NO bearing on the production cost, nor quality of the filter.
With that said, the SinghRay is a first class filter, I use one along with Nikon, B+W, Hoya and Hama CPL's. The reality of filter quality (not mechanical construction) is that any of the top end Hoya, Hama, B+W, Heliopan, SinghRay's, Lee, Tiffens etc are impossible to tell apart if a double blind comparison test is used.
I use Singh Ray filters, and B + W, and Tiffen, and Lee, and Hoya. Singh Ray produces some incredible and very innovative filters. But for something as basic as a circular polarizer, I very much doubt you will find a difference in real life usage.
I will not lose sleep wondering which is "better". The only one which I might be extra cautious is Hoya. Although I have never used a Nikon, knowing their excellence is making lenses, I will not have the slightest hesitation in using Nikon CP.