About voltage divider resistor network
Why they are so expensive?

Because: "laser-trimmed to extremely tight tolerances"


that takes time and expensive equipment
The market for such a product is small (mainly measurement equipment).
It is a product intended for use in professional equipment (i.e. expensive equipment)


Of course you can make such a voltage divider yourself ! If you have a way to measure the division ratio of your divider accurately you can tweak the divider to any ratio you want.

It is a similar process to what the Vishay product uses, there the resistors are made too low in value. Then they're laser-trimmed, with this they can only increase the resistor's value. So if you stop at the right moment, you can make any value you like.

You can  build a voltage divider and it will not be accurate. Now measure it and keep adding extra series resistors until you achieve your specification.