Topaz

These dragons can be bought with topaz, a currency that you can only obtain by using real money. All 5-star topaz dragons cost 500 topaz, which is 500 USD. Sometimes there are purchase events where one of the topaz dragons will be offered at a discounted 300 USD.

I personally suggest not rushing to buy any topaz dragons. Take your time to figure out if you need any of them. Many high-tier and paying players do not use them. I suggest only buying one when you have decided on a team and build you want to use and have all the other dragons in that team ready.

These items are also available for purchase with topaz. If you are not interested in the topaz dragons (or have all the ones you want), the next best purchase is the Legendary Selection Ticket - once you have bought the one you can get from the Level Package, of course.

The others are not as worth the money, although you may decide yourself whether or not it is worth it to you to buy the Skill Slimes and Enhancement Points - things you need in abundance. You may also wish to use them to purchase Capsule Coins when you are in need of the resources Capsule Draw can give.