I imagine it's legal almost everywhere as long as the basis for the segmentation isn't on a protected category (race, gender, etc.)

A lot of people have a reaction like yours: This practice sucks and shouldn't happen!

That means we don't actually need to make it illegal; since, as the blog post linked in the grandparent notes, "it pisses the heck off of people," the free market will take care of it for us if we're patient.

That blog post mentions specific multiple instances of companies that attempt egregious segmentation getting their lunch eaten by companies that don't.

As I said, the whole article is worth reading.

It seems to be legal in Europe, too (I'm not a lawyer).

Sadly, procurement departments at big corporations play the big discount game (their bonuses are tied to the discount they get): don't get surprised if they try to diqualify you if you don't give a discount.

In that case one could argue that the service is tailored to the meet the precise requirements of the client and it explains the price variation (and why the price isn't displayed).