As I and some others have mentioned, the card is much more balanced (but still a bit OP) if the cost depends on the total "power" of creatures with the name, where how to measure "power" is abitrary, and use some way to stop them rather than kill them.
For example, if the cost is the total attack of the creatures, and the cards stops them for 3 turns, then stoping 3 Ruby Dragons whould cost 45
, which is quite expensive. Think stoping 23 fireflies for 3 turns only at the cost of 69
, you would probably take bakc the idea of using it.