Pay attention to your patrons
If you are having a problem selling a plant, before you lower the price try doing a double check on the quality which means did you prune all the leaves? Is the plant healthy? Is it infested with bugs? These are all factors that can cause a customer to not want to pay full price for a plant.
Quality Soil = Quality Plants
The best investment you can make in the game is an upgrade of your soil. Always upgrade to the best soil that you can afford as quickly as you can. Higher quality soil will allow you to procude healthier plants that sell for more money.