You know how there are three appraisal methods, each with its own purpose? I think you can use two of those methods when evaluating landscaping.
You can look for comparable properties and adjust, not just for building features, but for landscaping.
You can use the replacement cost method, and get an estimate on what it would cost to replace the landscaping (A LOT!) Of course, you're not going to replace fifty-year-old Japanese Maples; but I suspect that the market value of established and beautiful upscale landscaping is at least 50% of the cost of landscaping a grassy wasteland of a lawn.
All the best,