贵

English says 'This is expensive,' so learners add 是 before 贵. Chinese adjectives usually act like verbs already, so 贵 means 'is expensive' on its own. Say 这个很贵 for a plain description. Say 这个贵 when comparing it with something cheaper.

很贵 just states the price level. It can be neutral, like reading a price tag. 太贵了 adds the idea of beyond your limit, so it often sounds like a complaint. In a shop, 太贵了 is the classic move before asking for a discount.

贵吗 is a plain yes-or-no question, like checking a fact. 贵不贵 repeats the word with 不 in the middle, which is also common in shopping. Both can ask the same thing, but 贵不贵 often sounds more conversational. Inside a longer sentence, 我想知道贵不贵 is natural, while 我想知道贵吗 sounds awkward. “贵不贵？” (Is it expensive or not?) ── [《问问题》](/read/asking-questions-daily-life#s7)

贵姓 is a fixed polite phrase for asking someone's surname. Here 贵 means honorable, like giving the other person respect. It has nothing to do with price. When someone asks 您贵姓, answer with 我姓... or just your surname.