Seven day and permanent mounts can be purchased either for Crowns or Gold in the Shop you access through the icon on your screen:
Just select the mount you want to purchase and above the BUY button are two tabs, one for Crowns and one for Gold. Select Gold and the prices will change to the gold values for the items.
Our good friend the Friendly Necromancer tells our fans all about it on this post: http://thefriendlynecromancer.blogspot.com/2009/10/how-do-you-buy-mounts-with-gold.html
