"That is quite expensive."
Translation:그것은 상당히 비쌉니다.
Here are the translations for "꽤": fairly, rather;
Here are the translations for "상당히": pretty, considerably, well, some, decently.
"꽤" is incorrect here because as clearly shown here, "quite" is not listed in the translations. Trust me I'm a Google Analytics and Translations consultant.
The issue here is that duo accepts "quite" as a translation for 꽤 in some of the 한국어 to English translations.
Weird because my korean girlfriend says it makes perfect sense. To her, 상당히 just sounds more exaggerated.