Who is 'self-sufficient'?
If you are an EEA national, you are self-sufficient if, during any period of residence in the UK, you are able to meet your own needs for accommodation and maintenance without receiving UK social assistance. The only benefits that count as UK social assistance are the means-tested benefits: housing benefit, income support, employment support allowance, income-based jobseeker's allowance and council tax benefit. There is no minimum income requirement nor is their any requirement that your needs are met solely from your own resources, they could be met by family or friends, or from a work pension, or some combination of these.
Even if you have very few resources, if you are managing without using means-tested benefits, you are likely to be considered 'self-sufficient'.