You can't really say that in general it is "30%". Is that for multiple sclerosis? What type of intervention?

Generally the placebo effect will vary depending on the type of intervention. A very novel and invasive intervention like this will have a high placebo effect.

Given that psychological treatment alone can reduce the fatigue in MS to below that of healthy controls (van Kessel 2008), it's likely that the placebo effect plays a large role.