The Black Market is a way for people to buy specific towers with Gems 
Where to find & Tower pricing
To open the Black Market, you must find the "Store" button on the left side of your screen. See the picture on the right for a visual reference of this button. -->
When you click on this button, a menu will appear in which you will see all available gamepasses to buy under the "Passes" button, all available in-game item bundles to buy under the "Bundles" button and lastly, the Black Market and gem-buying section under the "Black Market" button. See the picture on the lest for a visual representation of the Store UI layout.
Tower pricing
As you can see in the picture above under the "Black Market" section, each rarity has its own price. Here's a list with all the prices:
- Rare rarity towers cost 20 Gems
- Legendary rarity towers cost 50 Gems
- Mythical rarity towers cost 150 Gems
- Godly rarity towers cost 600 Gems
- Divine rarity towers cost 1200 Gems
- Ultimate rarity towers cost 4800 Gems
Note. you can buy Limited units (from seasonal events and the battlepass) in the Black Market for double the price of their rarity.
(Example: A Limited Godly unit could be bought for 1200 Gems )