Yes, more expensive and they generally assume the average owner isn't going to repair much so it'll last as long as the engine does. Granted, depends on what you spend an how old it is, some cheap decks today are thinner than they used to be.
You can get longer life out of a steel deck by spraying the underside with cooking spray (oil) before mowing if the grass has much moisture in it. That also makes cleaning it out easier, so you don't have wet grass sitting there, promoting rust. Push mowers, I'd just tilt them up (not enough to let oil leak out) and spray with a hose after mowing. If you want a cheaper alternative to cooking oil, use leftover old 2 cycle oil/gas mix but put a little more oil in it, and it'll still be thin enough to spray with a typical spray bottle. Do wait till the gas evaporates to run the engine, don't spray with a hot engine (odds of a problem are low but why rush it?).
If you've let it go for too long, you can do the same as any other rusty metal and wire brush it to a hard surface then rustoleum/etc enamel paint, but anything you do to keep water from soaking into crusty rusty metal is going to help.