There is no set minimum income.

The general rule is that they have enough income and/or savings above that at which they could qualify for any benefits. However, you may receive some non means tested benefits and still be self-sufficient.

See: Benefits and PR applications - UKCEN Citizenship and Residence for European Nationals and their families

This is from their guidance:

The Immigration (European Economic Area) Regulations 2006 (‘the Regulations’) state that a European Economic Area (EEA) national self-sufficient person or student and their family members must have sufficient resources available, so they do not to become a burden on the social assistance system of the UK.

When deciding if an EEA national and their family members have sufficient resources you must first check if they exceed the maximum level of resources which a British citizen and their family members can have before they no longer qualify for social assistance under the UK benefit system. If they do exceed the maximum level, then you must accept that they have sufficient resources.
You must prove that you have held CSI for the whole period you are relying on as a self-sufficient person.