Both are different adverbs. 꽤나: quite a lot 깨나: above/beyond a certain value/standard 돈 꽤나 많은 사람: a very wealthy person 돈 깨나 있는 사람: a wealthy person (more than above average, considered rich in the neighborhood / among friends) It's just a coincident that both are largely interchangeable while sounding similar as well. Note that 꽤나 refers to adjectives while 깨나 to verbs.