You need to know that buying the 3 marigolds from Crazy Dave every day is actually quite profitable: - you buy them for 2500$ each (-7500$) - on their first growth, you water them at least 3 times (+90$ because each plant gives you 10$ every time they are watered) and fertilize them (-150$*3=-450$, remember you buy 5 bags of fertilizer for 750$). Each marigold will produce a gold coin (+150$) - on their second growth, you water them (at least +90$) and fertilize them (-450$). Each marigold will produce 2 gold coin (+300$) - on their third and last growth, you water them (at least +90$) and fertilize them (-450$). Each marigold will produce a diamond (+3000$) - if you want to make them glow, you water them (+90$) and use either a bug spray or the phonograph. However, if you want to make this step profitable, don't use the bug spray but you can use the phonograph if you already bought it for an extra gold coin. - when the marigolds are fully grown, whether they are glowing or not, you can sell them for 3000$ each (+9000$).
In total, you can make a benefit of at least 3960$ out of the three marigolds.
Now, here's what you can do to make money quickly and easily: -if you didn't buy the three marigolds from Crazy Dave, do it and make them grow once. -change the date on your computer. You should notice that all your marigolds are already beginning their next growth and three new marigolds are available at Crazy Dave's shop. -buy Crazy Dave's marigolds, grow all the marigolds you have once and sell your fully grown ones. -repeat the last two steps.
You need to repeat this method at least nine times for it to become profitable but once you have four generations of marigolds in your garden, it takes about 2min to tend to all of them and you will earn at least 3960$ each time.
What I do is just feed my snail some chocolate and leave the zen garden running whenever I walk away from my PC. All of the fully grown plants will periodically pop out a silver coin as long as they are happy (i.e. they don't want water, bug spray, or the phonograph). I usually get $3-4K before the snail goes back to sleep, and that is with my garden only 3/4 full. It doesn't get much easier than that ;)