The scavenge cost of each card is equal to its mana cost
And thats the way I like to run it
I think in this case, the previous cards with Magmortar firing were deemed acceptable because they could be passed off a fantastical Pokemon attack, while perhaps this time TPCi thought that the smoke was too suggestive of a fired hand gun, which is a type of weapon that is more accessible to children than Pokemon fire or a mortar/bazooka, and changed the pose for that reason
to summon Gladiator Beast Murmillo
This makes it easier for players to plan out their decks since they'll know what monster types they can work with, and their cards aren't as reliant on what types of monsters their opponents have
Fittingly enough, the Duskull card has the move Come and Get You, which is exactly what these cards are showing