Four variables, one price
Generic imported bac water varies across four axes: sterility, pH, benzyl alcohol content and container integrity. Pharmaceutical grade product controls all four. The import controls the price.
Sterility variance is the obvious one. Less obvious is pH drift, which changes how compounds behave in solution. Preservative inconsistency shortens the usable life of a multi-dose vial. And container integrity, ampoules that crack in transit or vials with compromised seals, introduces contamination after the fact.
The shipping problem
Most import stock crosses the ocean in uncontrolled conditions. Heat cycles and pressure changes during transit degrade both the container and, in edge cases, the solution. By the time it reaches your bench it has been through more than the label suggests.
Domestic stock from a controlled warehouse crosses one state border in a parcel network built for it. The difference is not romantic nationalism, it is logistics risk.
Traceability, or the lack of it
When something goes wrong with an import, there is no batch record to check, no manufacturer to query, no recall mechanism. The vial is a dead end.
Batch documented product means every unit can be traced. That is not an academic nicety, it is what lets you rule the diluent in or out when troubleshooting.
The arithmetic of a compromised batch
Saving $3 per vial on a diluent while compromising a research batch worth hundreds makes the import the expensive option the moment anything drifts. Cheap bac water is cheap the way a cheap parachute is cheap.