The way I figure it is that by the time a player can afford the fountain, there isn't as much urgent need for money. So throwing some of it in the fountain will give the player something to do.

Rather than a windfall, I'd like to see the next sale come more quickly or some special effects: glitter/sparkles/rainbows, etc. over a customer or a plant. Or even "luck" when returning to the greenhouse in catching a rare insect.