This isn’t because they are basing it on AUM, but that there are more things to consider as assets grow. AA is easy early on, but as you age, you have to develop and implement a strategy and routinely balance. There are easy and more arduous ways to do this. You have you consider RMDs and Roth conversions. There’s Social Security strategies. Medicare planning. The list could go on.
I’ve done two fee only consultations with the same advisor. The first report was 5 pages. The second, 20! There was a LOT more to cover. Still, for a one time fee under $1000, from a published Boglehead…money WELL spent!