Mana costs can include specific colors of Mana, or generic Mana, which can be Mana of any color
In the Jace example, if there were two cards in your original graveyard and three cards in your newly-appropriated graveyard, that would satisfy Jace's requirement
However, it does suit well to blacks style of gaining life in big chunks to pay out for resources, which is what well be doing in big swings with Beledros Witherbloom
If you were to compare the prices of Lorcana cards to the prices of other games (Pokemon, Yu-Gi-Oh, Magic the Gathering etc.) you will find that they aren't worth as much
This is certainly one of the more obscure things to be protected from, but it fits into the game perfectly from a gameplay and theme standpoint