Guild Wars 2

Currently you can exchange

[250 golds for 2637 gems], meanwhile [2000 gems for 262+-]

You exchange your 250 golds and get 2637 gems, then you take 2000 gems out of 2637 and exchange it for 262+- golds.Repeat for infinite money.

But how does it really works, i currently played only 2 days so i can’t use currency exchange, but i want to know how it really works.

Currently
100 gems sells for 8 g 67 s .
100 gems costs 13 g 71 s to buy.

So it costs 13g71s to buy 100 gems. If you turn around and sell them, you will get 8g67s.

http://wiki.guildwars2.com/wiki/Gem
There is a 15% transaction fee whether trading gems for gold or vice versa. For example, exchanging 1g gives 85s worth of gems while reselling those gems returns only around 72s 25c , a net loss of roughly 28%.

Gems to Gold is roughly 72.25% (or 85% squared) the rate of Gold to Gems. Since it’s 85% squared it’s guessed that the exchange, whose rate is supposedly based on two internal pools, one of gems and one of gold, is sinking 15% of the gold going in and then coming out. It also makes it more difficult for a player to try and manipulate the exchange rate.

I should note that the gold you get from the exchange is from players buying gems with gold and the gems you get from the exchange are gems sold to the exchange.

You can use the published API to see the rates indirectly.

http://wiki.guildwars2.com/wiki/API:2/commerce/exchange