Since I am not in gametime selling business anymore I can reveal how to buy gametime for US/EU cheaper. Also you can gift it to your friends.
To access Russian store you need Russian proxy (and probably b.net account set to Russia, that is free to make, I did it without logging in):
To access Argentinean store you need Argentinean proxy (and also probably account country set to Argentina, just make new b.net):
Maybe its cheaper in Brazil, didnt check but I used Argentina as example. Now how to buy it. Its not as simple as it seems. Blizzard has very specific purchase system. It will allow almost every credit card / Paypal payment BUT after this order status is not Confirmed but Queued. After around 6-12 hours order will proceed and here is tricky part - system is very strict and your order can be rejected. It means gametime will disappear but your card/Paypal wont be charged (funds will come back on card in few days and on Paypal instantly). I will describe how to prevent it.
To get 100% working gametime this way best is obtaining B.bet Balance. After purchase from B.net Balance is made, order is automaticly Confirmed and your gametime wont vanish, order cant be rejected. Anyway. it maybe sounds easy but is not. Every time I tried to upload balance (and I tried a lot) it was rejected by Blizzard. We used to buy gold on Diablo 3, sell it via RMAH and get balance this way to get gametime.
Another option is proxy usage. You need 100% CLEAN proxy - it means nobody else can make any "black" purchase on b.net store from it in while your order will be Queued or it will be rejected. Obtaining such proxy isnt very easy task, after RMAH was gone I tried to purchase gametime this way using credit card but I succeeded only few Times, most of Times it was rejected.
Hope it helped you to understand how it works