This does not prevent targeting, it just gives a number a player must get over to do any damage. So would a cost = to gravity pull be about right?
I know; I'm asking for the
practical cases.
For example, compare Ruby Dragon + Strange Push with Ruby Dragon + Quintessence.
Because the opponent can deal over 15 damage in one spell (bolts), and because the opponent could infect and lobotomize it, it could be weaker than Quintessence.
However, because you can target a Ruby Dragon with no ability, if this costs less than Quintessence, then this combo itself may very well make Strange Push situationally OP.