This is an interesting and important question. I too have looked for some kind of standardized fee schedule for landscape architectural work and found very little. Architects typically charge a fee of around 7-15% of the construction budget. Sometimes this is a sliding fee; Lower dollar budgets have a higher percentage and higher budgets have an inversely lower percentage. I use something similar in my own practice except on very small jobs that I bill on an hourly basis.
I would be happy to share what I have with you.