"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)