I'm from Spain, so I'm quite sure of what I'm talking about. Spain might be a highly developed country, but its gdp per capita is not too high compared to the likes of Germany, UK, France, Denmark, US, et al.
Basque country is, again, rich because it has an special tax treatment and could be considered a tax heaven to some extent.
The Basque Country has a GDP (PPP) of 108 [1]. Higher than France (101), as an average, or Italy (97).
I'd say its a well developed region comparable to Northern Italy. Both were badly hit by the Euro. Had they kept their own currencies, they'd be closer to Sweden or Southern Germany.
Basque country is, again, rich because it has an special tax treatment and could be considered a tax heaven to some extent.