Not exactly double. The cost drops with the more you purchase.
4m costs 1400 gems, 8m costs 2467 gems. But yeah in your case there is little difference between a 50% fill and a 100% fill since your storages are almost half full already.
Look at the screen again the next time you upgrade an expensive building. It'll make more sense then.
Also the wiki has a gem calculator.
http://clashofclans.wikia.com/wiki/Gems#Calculators

