"Silver is expensive, but gold is more expensive."
Translation:Silber ist teuer, aber Gold ist teurer.
Why is it "aber Gold ist teurer" instead of "aber Gold teurer ist"? I thought the verb had to go to the end of the sentence in subordinate clauses.
It is not a subordinate clause. There are some conjunctions (like "und", "aber", "denn") which lead to a full main clause adjacent to the first one instead of a subordinate clause.
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)
weil Steigerungsformen von Adjektiven auf Deutsch immer mit Endsilben gebildet werden, nicht mit davorgesetzten Worten.
I am very upset due to the fact that I got it wrong and then the right answer was the exact one I had written and it said it was wrong!
Would it be wrong to use sondern rather than aber in this sentence?
No. "sondern" only fits if the first part of the sentence is negated.