I have found that if you wait longer to harvest your plants, they become more valuable. I have a money garden, as I call it, because I only sell everything in it, I don't really cook with the items and I make more than 200000 simoleons in one harvest. My garden is mAde up of all alien plants and sometimes each fruit will get me 700 simoleons(they are raised to perfect quality). I wait until the plant is full and the harvestables become small. Also after I leave the lot and come back to it a little bit later the value seems to go up too. I will post pictures sometime to better show what I mean but I hope that this helps some aspiring gardeners.