I don't think it's unreasonable that for many people, a publisher with experience and an existing distribution network & agreements, perceived authority as a "real" publisher, and a marketing budget and plan will result in 10x the sales. I've had a friend self publish in another industry and getting books shipped around the world and in stores is a hassle as a lone operator - and that's if people even know you exist.
If you live in the US (she didn't) then Amazon can be a decent option for a lot of reach though.
Amazon and other similar print on demand services also help with this but it's not suitable for all products (full colour can be a problem).
If you live in the US (she didn't) then Amazon can be a decent option for a lot of reach though.
Amazon and other similar print on demand services also help with this but it's not suitable for all products (full colour can be a problem).