Cassettes on bikes that are used often as long as the chains are well lubricated almost never rust. The lube from the chain coats the cogs of the cassette inhibiting rust. Apart from that, more expensive cassettes may use various dodges to reduce weight, things like alloy carriers with several cassette cogs riveted to them, titanium cogs for the larger sizes, even aluminum alloy cogs for the biggest sizes. Usually, cassettes are designed to last on average about the same time. Lightweight components cost a whole lot more, but they have a reasonable lifespan compared to less expensive heavier components. Nobody would spend twice or 3 times as much for something that wears out in half the time