$800 from a magic plant - the second one I discovered (I've only discovered 2). But I only sold one at that price after waiting about 5 minutes, and my nursery was full of them. They MAY continue selling for that price if I was patient enough to wait it out, but putting it down to $700 gave me an empty nursery within 10 minutes, and a nice bank balance
I grow 12 of those every few "turns" now to top up my money, then spend it all on fertilizers and potions to keep experimenting for the next few turns.