This isn't a bug but more of an issue that I don't think people are going to buy luck potions at their current price. This includes not just luck potion itself, but the 2 greater versions of it. Luck potions should, in my opinion, cost FM, not Gran. Having them cost Gran and not FM seems just silly. All a luck potion does is slightly increase your chance of getting into a treasure battle for your next 10 fights. So your paying Gran, which you could have just spent on buying cards, to give yourself an increased chance of maybe getting to compete with some one else for a card. Even if you buy the most expensive luck potion which gives you an increase of 50% of having a treasure battle for your next 10 fights, said potion costs the same amount as a pack of 5 cards and those 5 cards are guaranteed while treasure battles require winning so even if you have 5 treasure battles over your next 10 that you wouldn't have had with out the luck potion, you'd have to win all 5 of them just to break even for what you would have gotten just by buying a pack.
If you really don't want to make them cost FM at the very least it seems like the Gran cost should be significantly lower then it currently is to make them worth while. At least that's my observation.

