Warsaw.
Own 25 m2 flat: 600 usd
Cheap meals and fair price in good restaurants.
Best in Europe public transport 25 usd/m.
In general more than half cheaper than Berlin.
I working with big graphs. Over 1 bln entities & 40 bln edges.
Best open source graph database is ArrangoDB they have master to master cluster.
Fastest and best technology for graphs have commercial TigerDB, but you must pay >300k annually.
Networkx is great but loading full model - but for what you doing should be enough. :)