Concept:Mana costs for monsters in MTG
From D&D Wiki
Monsters and color/mana cost
Celestials, Gold dragons, air elementals
Anything aquatic really, blue and bronze dragons, Water elementals
Beasts, Plants, Fey, green and copper dragons, earth elementals
Fiends, red and brass dragons, fire elementals.
Undead, oozes, black and shadow dragons.
Humanoids and giants will be classified by flavor/class. Monstrosities will be multicolored.
Mana Costs for Monsters
Okay so summoning monsters is going to go by CR. so lets say that i wanted to summon a red dragon wyrmling, It has a CR of 4, and its a red creature, so its mana cost would be four of any color and a red mana. The formula for mana cost and creatures should be as follows:
CR of the creature in mana of any color and of the creatures color, then 1 mana of the creatures color for every 5 CR.
Creatures with a CR of less than 1 have a mana cost of one mana of the creatures color.
And for the sake of not breaking the game, id say only keep the CR of the creatures to 20 and below.
When summoning creatures of a CR of 1 or less, you may summon two of a CR of 1/2, 3 of 1/4, and 4 of 1/8.