"The art from the Golden Age is the most expensive."
Translation:De kunst uit de Gouden Eeuw is het duurste.
Why DE kunst and HET duurste? Is the second article not refering to kunst too? Please somebody explain!
I think it should be: "de kunst (...) is de duurste" or "de kunst (...) is het duurst", because the first sentence could be completed: "de kunst uit de Gouden Eeuw is de duurste kunst." In this case there is gender congruence between the subject and the subject attribute. The second sentence can't be completed, but I can't explain why (I'm a native speaker, but haven't studied the language).
The use of prepositions depends on the context. Even for languages as close as Dutch and English they often cannot be translated 1:1, also see this explanation.
Uit is used because it is about a time period. In this kind of context van is used to express ownership or the creator:
Het schilderij van Van Gogh uit zijn vroege periode is nu van het Kröller-Müllermuseum. = The painting by Van Gogh from his early period is now owned by the Köller-Müller Museum.