On landscape designs, I use another landscape/nursery to do all my design work.
They charge me, so I charge the customer a marked up non-refundable price.
Even if we don't do the job they paid for landscape plans and design.
If we do the job, the designing work is an extra cost.
This way I don't loose any time, and still get paid for time I contracted out.
So I guess to answer your question, charge them your hourly rate for the designing process. You are still spending time on the job. You're working right?
The place I use charges me $60 an hour plus blue prints and I charge customers $75 an hour plus blue prints.
They sit and discuss the layout, and I give bids off the finished design. I am not at the proceadings untill the design is complete.
Good luck dude