Probably €15+ a pair. I expect to pay £15-25. I buy mine from outdoor sports brands which might not be the style you're going for but DeFeet and Rapha make good ones that should be available in the EU vs some of the US recommendations.
I wear taupe coloured Smartwool™ light hiking socks. Colour is fairly versatile and they are both warm in colder conditions and breathable in hotter ones. Also quite thick, so they shouldn't wear out too quickly
Calze sabas are the best socks money can buy in the EU in the price range up to 30€. 100% wool if you want. Or a bit more durable blends as well. A pair is around 10€ if you use the newsletter code.