To anyone ever playing the full game (as the demo is too short for 'strategy')...
I'd recommend only buying extra seed slots with the money you earn. None of the other stuff. It's pretty worthless.
If money seems slow, go play some mini games or puzzles before you proceed. They're fun and cash will come rolling in fast.
Raising your (free) zen garden plants is definitely worth it, at least I think it is... didn't check expenses for it.
Crazy Dave gives you like 2500 and 3000 dollars for fully grown plants.
PS: type 'future' to give the zombies funky sunglasses ('zombies from the future').
Tree of Wisdom told me that. Damn money eater he is! And not that much useful info coming out of him.