Professional design typically starts at around £5,000 for a smaller plot. This includes full 3D cad design, around 3 rounds of design development, and leads to a completed pack suitable for construction costing, with general arrangement plans, set out plans and detailed materials and planting schedules, lighting and irrigation designs.
Larger and more complex gardens, particularly those working around new builds obviously need a great deal more detailing and input and need to be priced specifically to ensure we capture all the work you want (but no more!)