Bilbao is relatively expensive. In fact the most expensive city to live in Spain is San Sebastian that is also in the vasque country. Barcelona is the second (or the first capital more expensive) and Madrid is the third but this is just an average. It depends heavily on the street and zone and how expensive is to reach your working place from your home.

Cáceres, Lugo and Palencia are the cheapest (but the tech scene should be close to unexistent in those places).

Strongly turistic areas closer to the sea are more expensive to rent a place in general terms, specially in summer, but can be cheaper in winter and off-season