You're laboring under some misconceptions. The ISS was expensive to build for the same reasons many things are expensive to build, it used new technology in a new configuration and was essentially a one-off product. It is a functional prototype.

There's also some in-space capabilities that have been lost with the retiring of the Shuttle. There's not really replacements for those capabilities on the horizon.

For starters the Shuttle was designed as an orbital work platform. In its construction configuration it launched with the CanadaArm, an airlock, and whatever ISS module it was delivering in the cargo bay. This was all in addition to the OMS, power and life support, and the crew cabin. It could fly modules to the station and assemble them without using resources of the station itself.

The Shuttle as a space truck facilitated one of the best aspects of the ISS design: the integrated truss structure. The truss structure of the ISS contains most of its base infrastructure. It mounts the solar and radiator panels, power and coolant distribution for same, and other support infrastructure. The individual crew modules on the non-Russian segment of the station don't need to use their mass or volume carrying these support elements. This leaves more of their interior envelope for astronauts, supplies, and scientific equipment.

The Shuttle also facilitated dedicated crew modules. Because the Shuttle's engines delivered modules to the station and could be placed with the CanadaArm/Shuttle crew they did not need to be mated to an independent spacecraft bus.

Compare the non-Russian modules of the ISS to the Russian segment. Because the Russian segment is essentially two docked independent spacecraft there's a lot of dead weight and needlessly redundant support systems. Each of the Russian modules have their own power and cooling since each needed those things before they were part of a larger station.

The Mir and Salyut stations, from whom the Russian segment of the ISS are derived, had the same sort of redundancy. The Mir was crowded and had very limited space and power for experiments. The modules also did not have the ability to support each other. If the power or thermal systems broke in a module it was dead and needed to be sealed off from the rest of the station. Because the modules were independent spacecraft they didn't have exterior connections or serviceable parts that could be repaired with a space walk.

Without the Shuttle or permanent orbital work platform like it, near term space stations have to be constructed out of docked independent spacecraft like Mir. A bunch of Dragon craft mated to a central docking module has far less capability as the ISS truss/module design.

It would be possible to build an ISS-like station without the Shuttle. The SpaceX Starship could be given a cargo bay, remote arm, and integral airlock. It could do a pretty good job configured such but that is not currently something SpaceX has announced. It would also require a lot of development effort as cargo bay doors are a structural weak point in addition to who knows how many changes to the Starship's design. Even a couple mated Starships in a "wet workshop" configuration would make for good orbital stations.

Assembling things in orbit is hard and unforgiving. Don't just assume that a reusable launch vehicle magically makes it cheap or easy. Launching shit into orbit is relatively easy, it's at least a solved problem. Docking that shit to other shit in orbit is much harder. Designing and building all that shit is expensive. Keep in mind, if your toilet breaks in space it could kill you if it can't be repaired or replaced quickly. So everything from the airlock to power distribution to toilet needs to be well designed, tested, and built or it is likely to kill you.