Pro CAD programs are going to run into a lot of money. If you plan to do this professionally, it may be worth the investment.
If you want to $50 consumer version, you may want to look at Sierra's "Complete LandDesigner."
Some Excerpts from a Software Review...
"For most of us, unless we hire a landscape architect, the home's landscape evolves. You have a general idea of what you want and then you add trees, shrubs, grass, ground covers, etc. Over time you'll add and remove what you think works in your landscape. The premise behind software packages like Complete LandDesigner is you can see a representation of what your landscape will look like before you ever plant a thing.
The Complete LandDesigner two CD package includes:
3D Landscape
Photo LandDesigner (you can import a photo of your home and then landscape around it using drag and drop.)
Garden Encyclopedia
3D Deck and a user manual.
I read most of the manual and ... zeroed in on 3D Landscape to see if I could design my side yard using the program.
... you'll find Complete LandDesigner is quite a package for the money.
The first time through the program offers video and guidance to help you along your way as you draw out your property boundaries and a representation of your home. For me this was the most difficult and I never did quite get my house to look right with its long porch, 8 foot sliding glass doors and sloping property.
You also need to add driveways, patios, walls, walks, etc. The more willing you are to explore the program the easier it'll become. I spent a couple of hours and ... printed out a black & white top view of my proposed landscape that was actually kind of neat; a 3D view that I wouldn't write home about, and a price list for the things I would need to complete my landscape.
There's also a design guide that suggests different garden ideas along with site preparation and more. The program will also connect you with the Sierra Web site so you can download some add ons for the program such as more houses, additional trees and shrubs, fountains and spring flowering plants. "
I'm sure the pro programs run rings around this one, but hey, for 50 bucks... it'll let you get our feet wet pretty cheap.