User blog comment:Dodo8/About Unique Dragons and Dragon Spells/Potions/@comment-3271569-20120907121725

It's seems it's not really random:
 * 1) Free: In most casses you get an potion
 * Med: I think you will get an potion this way
 * 1) 100%

It might be you get another potion on free the 4th time but I'm not sure. Funny enouge I don't pay for potion and I always finish with 3 potions. It's really impossible to get an unqiue dragon for 10 potions. It's only 3 days. That are 72 hours so 12 times. Also your potions will be reset every week and that's why I wouldn't spend gems on it. They are just useless if you won't spend them in time.