From Diablo Wiki
As currency can be used to purchase gold, which can then be used in the GAH, the line between the two auctions houses is blurred and the prices generally work out equivalently, once you factor in the fees and transfer costs. (The exception is at the very high end, since RMAH item sales are capped at $250. That works out to about 1 billion gold at the minimum gold price of 1m gold for $0.25, while top items sell for up to 2 billion gold in the GAH.)
There is no RMAH for Hardcore, and the RMAH is legally-prohibited in some regions (such as Korea). In those markets, the only supported Auction House currency is gold, though of course black market sites exist to fill that gap in legally-provided gold sales. Another complication comes from the fees for withdrawing currency from a Battle.net account, since there are considerable fees (15% i most regions) charged by PayPal and Blizzard. These costs are much higher for many international transfers, especially those to Asia. This has largely priced the so-called "Chinese gold farmers" out of the Diablo III market, since their lower production costs are offset by the high costs of transferring the gold.
As a result, despite the official in-game support, gold can be bought through black market sites for around a third the price paid on the in-game Auction House. This is not recommended, since it's a risky and unregulated market that exposes players to the risk of being ripped off, having their account hacked, and other unfortunate outcomes that Blizzard constantly warns players about.