Rent is generally very cheap, 250EU/mo for a central apartment (12mo lease). I am staying here only a month however, so am paying a lot more.
Eating out, beer etc is very cheap. Numbeo gets it about right.
Amusingly, most Bulgarians I speak to ask why I would want to come to Bulgaria of all places. I guess it's like many places: great fun to visit or live for a few months but painful for long-term residents.
Upsides apart from cheap living: it's a fun, walkable city with a lot of excellent parks and very little traffic.
Downsides of living here: shockingly corrupt bureaucracy.
That said, I'll head to Romania when my month here is up. I lived in Istanbul previously, so let me know if you want to hear about that too.