You cannot pay for it. Dark Star Cho'Gath was an exclusive Charity event skin that was only available for a very limited time (because of the charity). The only way to get the skin right now is through 10 Gemstones (the purple diamonds). There's no other way, sorry. :c
There's probably also the small chance to get it from Hextech Crafting (like the Hextech skins), but that's so unreliable it may not even be worth mentioning.