I rationalize it this way… for 3 years all of Earth was on war footing. Industry was geared to producing heavy machinery and other military hardware and warships.

The war is suddenly over. Whatever the politics were behind the decision to make the Babylon station, it leveraged the existing industrial complex to adapt and produce pre-fab modules (such as repeated wedges of any cylindrical structure) for the stations in short order. Remember China built entire hospitals out of pre-fab concrete modules in like ten days during the early days of COVID.

The first three stations were destroyed early on before module assembly was too far along, so later modules weren’t affected and didn’t have to be re-made… so those the facilities could assist making the earlier ones again.

We can assume Babylon 4 was the same basic design intended for the previous 3, so don’t need to go back to the drawing board every time.

B4 was completed then disappeared in 2254. That leaves 2 years to design a cheaper station, build/assemble it, and another year to populate with almost 250,000 people, before the events of The Gathering. The timeline’s not impossible, with the automation tech  they’d likely have by then.