I would make the crops grow much slower, maybe as much as 3x slower. and the return value is pretty high, you buy seeds for $2 and get $5 for every crop you sell. I would make this gap smaller.
and then to add more stuff into the game, maybe you can add a fertilizer power-up that temporarily makes your crops sell for a higher price and therefore give you higher return.
and then buying a bigger plot would also be cool, as WickNewbie just said.
(I like the isometric design)