it doesn't tell you the gravestone cost until the plant's turn is over. So it is not that easy to predict.
zombies have deadly, in fact zombies can combo deadly with strikethrough. I mean team up is as fair as deadly imo, so it is basically a wash.
edit: forgot to mention. Zombies have better creature stats in general than plants, and they have frenzy, so that's how they balance out team up really.
