How do you cross polinate plants?
You drag the pollen from plant a to plant b. Both plants must be "mature" before you may start the pollination process.
How do you get magic plants?
You get magic plants by correctly cross polinating 6 plant combinations that the game has predetermined to be "magical". You can see exactly what plants you need to cross polinate by clicking here.
How do you know how much to charge for plants?
The only way to really know is the just keep trying different amounts. When a customer walks up to the plant, click on him or her and he (or she) may give you an idea on your prices are for that paticular plant. If you have charged to much money for your plant then you will see this little bubble over his head.
But keep in mind that just because one person doesn't like the prices of your plants doesn't mean that someone won't eventually buy it. If you have enough time, try and keep the plant at that price. Perhaps you will get lucky and find a buyer. However if the plan doesn't sell within 5 minutes then it's probably time to lower the price.
If you aren't charging enough for your plants sometimes you will see customers pop up with the happy money bubble over his or her head. This typically means you can raise the price of this plant.