Yes very expensive like balletbooter said, one pair I saw was £400 (£800) which is astronomical! But I found one pair here that I would get 12" heels on:
http://www.nadiaheels.ch/en/contents...d=38716.434028 which I quite liked - £100 ($200). They're a bit plain I suppose, but I would get them in pink (I like the striped style of my other platforms I linked above)
I have bought platforms for about the same as you derf but I expect if I go over 8" I will be expected to pay a premium as they're so rare.