Make the prices that are based on how much people sold of that plant individual. I've just sold 6000 cannabis and it's made the price go down to around $6/per from $20/per because of me. All of the new players now cannot make money from it.
Also, make the weed killers recharge at a cost when you click them or something, because it's awkward for some people to get to their weed killers if they've placed covers over their crops to stop them getting stuck in them.