The problem isn't that the payouts are bad for plants, it's that the time played causes the zombies to make a lot more. If the plants get a perfect game and shut out the zombies on the first garden they only get so many kills and only play for a short time resulting in way less coins. If the zombies play thought and win, it's a long drawn out game and they make 6-8k coins JUST one kills and other stats. The payouts for gardens taken/protected are very similar and it really wouldn't be fair to anyone if the plants could play for 5 minuets and make 10k coins but the zombies have to sit down for twenty minuets to make the same.
Yeah, I do think that the coins need to be balanced but just jacking up the payout for plants isn't a simple answer like people think.