well about the cheapest gold you're gonna find is 100g/20USD
but the prices change depending on whats going on in wow, such as the bc coming out making the prices of gold go up just check different sites they are all diferent and they have holiday specials as well.
err.......blizzard has banned many farmers' accts, wow gold price goes up, and many site can not deliver gold on time.
pay more attention on their delivery time than price.