IME, there's no reason to believe that replacing chains early saves any wear on a cassette. how could it? the chain is still rubbing on the teeth of the cogs. think about it for a minute or two.

IMO, to save money on drivetrain components, clean the chain regularly and buy steel cogged cassettes. not advice i always follow WRT cogs. but all in all it's not much of an economical burden IME.