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There's a Sigma 100-300 f/4 going on eBay UK:
Sigma EX 100-300mm F4 APO IF in Pentax K Mount | eBay
It's up for £500 at the moment. If I had £500, I'd snap it up, but unfortunately I don't, so hopefully someone else can get a good deal on this ridiculously rare lens. Though I highly doubt it'll stay at £500.
I've not checked the pictures of the lens, but they say it's in very good condition, though they call it 'demanding'.
Someone can grab a deal? Perchaps?