Is the Zen Garden a good investment?
Solution 1:
If you are actively playing the game, there's no faster way of getting gold than harvesting it through playing the last stand mini game (and you can still go semi afk on that minigame).
The only reason why you would want a good zen garden is to be able to harvest the coins while you are completely afk. Once you have fully grown your plants and made them happy, they will randomly drop coins over time and your snail can then collect them (give him a chocolate to make him fast for a while to increase the rate).
Here is a youtube video of how to do so:
Solution 2:
I'd say the Zen Garden is a good investment. If you give Stinky the snail a piece of chocolate, with a reasonably full Zen Garden you can easily gain $10,000-$20,000 (or $1,000-$2,000 without chocolate) without doing anything.
On the PC version you can just leave the app running in the background. On the iPhone version you don't even need to be running the game, just come back an hour later and it will say "Stinky picked up N coins before falling asleep".
Once you buy the gramophone you get infinite use so you can get your $15,000 back pretty quick. Bug spray is also fairly cheap compare to how much money you'll get back.
You can ignore the night garden for now - the benefit is small since you can only store 8 plants there.