[0] For one thing, сигурно ще трябва да научиш български, which doesn't sound like a great use of time.
Monthly rent €450: https://www.property.bg/property-67234-fully-furnished-house...
Both are enough for bills and food, or at least seem to be (I don't know how property taxes work in Bulgaria).
And given that figure I gave you was average salary and thus won't cover kids or pensioners, mostly of the people at or below that threshold will be supporting a family with that money, not just themselves, and that's the main reason they won't be living alone.
FWIW, I could manage on €10k/year even though I live in Berlin, but I'm (a) weird and (b) also living with someone in an apartment. Still more space each than if we were trying to live on a train, though.
Back in the UK, you can also find cheap actually-a-house houses despite the nonsense that is UK housing: https://www.rightmove.co.uk/properties/144969266#/?channel=R... (£380 ~= €444 / month) — I think £332/month should just about cover one person's food and bills, including council tax, in the UK, so I'd be surprised if one couldn't do at least that well in Bulgaria, where 5% of the population have an income of €208/month or less.
I don’t see how that can be true if you want to live alone. Maybe in some of the poorer countries outside the EU.