Skylab was cheap(er) because it was launched in one single stage (IIRC it literally was a Saturn V cargo bay). The ISS was designed to be built up over several

Skylab was cheap(er) because it was launched in one single stage (IIRC it literally was a Saturn V cargo bay). The ISS was designed to be built up over several years, over several launches, with each launch costing much less than Skylab but obviously more overall. It was a design decision that favoured amortizing costs for an age that couldn't afford another Skylab.