If you keep your drive train clean the wear on the cassette will be much less than the chain due to stress and its contact with the front chain ring in addition to the cassette. I will go 3-4 chain replacements before needing to replace the cassette or parts of it. Now I am a bit obsessive and will clean my chains after every ride and on occasion every two rides. I do this simply to prolong the life of the components. Don't get me wrong I like new parts but I also like my parts. And it feels better when I replace something because I wanted to and not because I had to.