The introduction of a way to use real money to buy virtual cash for World of Warcraft has prompted a big change in the value of the game's gold.

The exchange rate for dollars fell by almost a third on the first day that Blizzard let people swap real cash for game gold.

At launch, players could spend $20 (£13) to get 30,000 gold coins to spend on gear in the fantasy game world.

But 24 hours later the same amount of cash netted players about 20,000.

Before now the only way that World of Warcraft players could artificially boost the fortunes of their characters was by visiting a grey-market site and surreptitiously buying gold from unlicensed vendors.

Fell sharply

The practice was fraught with peril because it was technically a violation of the WoW terms and conditions. Anyone caught buying gold this way could have their account closed down.

Late last year Blizzard announced plans to introduce a $20 "game time token" that could be converted into WoW's internal currency. Currently only North American players of WoW can buy the token.

The tokens can be traded on the game's internal auction house for gold - effectively giving people a way to turn real money into virtual cash. Those with lots of WoW gold can buy the token and use it to pay for their subscription to the online game.

The token trading system went live on 8 April and initially the exchange rate for each one climbed past the 30,000 starting point. But within hours the dollar exchange rates fell sharply and are now hovering around 22,000 for $20.

Some speculated that the dollar exchange rate would fall further to reach those seen on grey-market sources of WoW gold, which currently offer 10,000 to 15,000 coins for $20.

'Lot of fluctuation'

Before the launch Blizzard said the exchange rate for tokens would be set by internal game metrics.

Alec Meer, of the game news site Rock, Paper, Shotgun, told the BBC that there had been some initial "overreaction" to the drop in value of each token.

"It doesn't spell doom," he said. "Blizzard is trialling something new. They went in high to see what would happen, and there's going to be a whole lot of fluctuation as a game with a population of several million adapts to it.

"I'm sure the long-term plan is simply to bring more transactions within Blizzard's purview, and in doing so potentially reduce the influence of gold farmers and keep people playing for longer," he added.