I did Ribble because they had something else I wanted and couldn't get a good deal on locally, so luckily I looked up GP4000 while ordering... probably should've bit the bullet and ordered 4 total, but then I'd never get around to mounting up the Gatorskins I have sitting around...

For US shipper, I found Planet Cyclery on eBay has a good price, $87.95 + free shipping for a pair of GP4000S (boxes not included to save on shipping). If you want the boxes, add $8.73 for shipping. If you want 10 tires, Planet Cyclery has a deal $429 shipped, and there's also the option to "make an offer" to get a lower price too. I haven't bought tires from them, but I did buy something else and it was shipped very quickly (ordered weekend and shipped out Monday) and arrived quickly too (arrived Wednesday). Edited to add: I think this is the same company as the one selling on Amazon.

I like to support my LBS, but $75 each is too rich for my blood... I do buy other stuff there tho!