Coins are the currency in Plants vs Zombies 2, and they are used to buy the Power Pinch, Power Zap and Power Toss power-ups, mainly. Early on in the game you can win very reliably without using these, but as you get into the later stages of the game, especially the Wild West stages, it will become ESSENTIAL for you to have enough coins to load up on the power ups, or else you won’t be able to win. You’re not going to beat the game without having a large number of coins.
Getting more coins is fairly easy. Go back to the stages that you can beat easily, and do some coin grinding. Each time you play, zombies that you kill will drop coins. Silver coins are worth 10 coins apiece, while gold coins are worth 100 coins apiece, and blue diamonds are worth 500 coins apiece, so you’ll want to go to stages where you tend to earn more gold coins. Level 6 in Ancient Egypt is an excellent example of this. Blue diamonds, though, are totally random.
You’ll earn a large number of your coins from the bonuses you get after you complete stages. A money bag filled with silver coins will drop at the end of the stage, and for every lawnmower that you have left over (they’re right behind the back row of your lawn), you’ll earn a silver coin as well.
Whenever the Yeti appears, go to the stage that it’s at and defeat it, and you’ll earn big prizes, such as blue diamonds. You have to kill the yeti, then beat the stage – if you don’t beat the stage you don’t get the prize. However, if the yeti runs off, you can restart the stage to try again. It’s recommended to use the plant food whenever you see the yeti, or to stick a Wall-Nut behind it so that it can’t run away.
Other than that, the only way to get coins is to buy coins, so it’s your choice: spend time or spend money to get the coins. Either way, you’re going to need the power-ups later on.