Pro Landscape is $1300 US, and is one of the best for the money. You can buy homeowner software that is much cheaper and is much less of a program. Pro Landscape is CAD based which explains the price and difficulty in using it. If you do not do a great deal of design work, or have delusions it will make designing easier, you'd be better off finding a local designer and working with them. Many a software program sits on a shelf because of the difficulty in mastering them, and that's a lot of money to be sitting around.
Kirk