It's free. The only cost is buying the game(s), either at a retail outlet or through the online store. The online store also has other things - extra character slots (if 4-8 isn't enough), unlock packs (for PvP characters who don't want to spend time unlocking things manually), and, by the end of this month, a special Bonus Mission Pack is going on sale.
But all of those are optional. You can have a very satisfying experience with even just ONE of the GW titles (although the more you have, the more you have access to). To play the game itself (once you've bought it, installed the client, and created an account) is free. FOREVER. FOREVER!!!
Also... buying money? What kind of game makes you pay real money for fake? I mean, some GW players try selling gold, but not only is that not endorsed by the company, it is against the EULA and is a bannable offense for both parties. I can't imagine the kind of company that would want to sell their own gold directly. Who needs to buy money when you can kill, loot, and sell the loot - the same way RPGamers have been making money for at least 20 years!
"As the good archmage often admonishes me, I ought not to let my mind wander, as it's too small to go off by itself." -Danilo Thann