"Silver is expensive, but gold is more expensive."
Translation:Silber ist teuer, aber Gold ist teurer.
There isn't much of a reason other than that "teuer" is a big irregular in that the second -e basically disappears when "teuer" is declined or put in the comparative.
teuer (expensive) - teurer (more expensive) - am teuersten (most expensive)
ein teurer Wein (an expensive wine) - eine teure Wohnung (an expensive apartment) - ein teures Buch (an expensive book), etc.
It is really just something you have to learn, but it might help to remember that the second -e is really only present in the base form (teuer) and superlative (teuersten), but is omitted everywhere else.
(In fact, I believe it is more for pronunciation than anything else - it's much easier to say "teure" than "teuere", I feel anyway)
Although the usage of articles differs in some cases, usually it is very similar to English.
So why should it be "das Silber" and "das Gold", when it is not "the silver" and "the gold" in English?
You probably think of things like "die Zeit" ("time"), "die Natur" ("nature") or "die Liebe" ("love").
This is one of the major exceptions. All those are abstract terms.