I'm a big believer in charging for your time. My first consultation with customer is free. If I have to come back for design purposes (if I can't bid off of a straight-forward project, like a lawn install) I charge an hourly rate for all phases of design. I offer a discount or rebate on a portion of the design cost depending on what they install. This is all determined at the end of the project.
I see your landscape program as a great sales tool. You might use it to convince them that you're designs are worth paying for. We use our portfolio of pictures and letters of recommendation and past client surveys to convey the message. A picture (particularly of their house) will speak a thousand words.
I wouldn't give them your picture/mock up for free. But it might also be the foot in the door to having them pay for your professional design service. Could you charge $75.00 for a picture of the front? Architects/designers charge for renderings of the landscape plan. You could too. Only you're using it up front in your presentation... so I don't know how you'd work it into the price up front. Bottom line - get your money for it one way or another. Either figure it across all of your jobs as a selling expense (at another $10.00 an hour to your design fee) or charge them per rendering/picture. Or, use the picture to get a retainer of $500 for the upcoming design. Build the picture into the design cost elsewhere.
I think you've got a great idea! Good luck. Let us know how it works out for you.
Lawn Lad, Inc.