A 16 year study by Vanguard (latest report from July 2018) has found that this figure is about 3% net. 

This is over and above what might be generated by non-planner assisted investment activity.  This means that even for small investors, a financial planner will not only pay for themselves but provide the professionalism to navigate the confusion, performance and peace of mind you might not get elsewhere or if you were doing the job yourself.

A recent example:  An investor was using three stock brokers and each one thought the others were managing issues such as tax.  When a financial planner was engaged it was realised this investor had lost around $200,000 over a number of years.  This investor only has a modest portfolio.

Source:  Peter Graham
PlannerWeb