I agree toblix.
I bought the first game for my Mac, then on iPhone and iPad, so I don't have a problem paying for it, but $3 for a plant just seems silly.
If the had a $20 for the lot pack I might be tempted, but there is a reasonable chance that some of the plants would make the game substantially easier (like the potato and torchwood), which means I would be paying to make a game that is already too easy even less challenging.
I feel like Valve got it right with DotA2 being free to play and then charging for cosmetic tweaks, but PvZ2 just left me feeling a bit dirty.