Politeness marker.

Yes, 貴 guì literally means 'expensiv, dear, honorable'; here it implies "[your] honorable company"; hence a politeness marker. Likewise, if asking someone's surname in a very formal situation, you can say "您貴姓 Nín guì xìng?", literally "you [singular, formal] honorable surname". (I prefer to say "surname" rather than "last name" because the Chinese put their surname first.)