The idea of volume discounts works for manufacturing. You design a widget and build a widget-maker for a million euro. Raw materials cost 10 cents per widget. If you make a million widgets, the cost of manufacturing is one euro and ten cents each. If you make two million widgets, the cost of manufacturing falls to 60 cents. Although the reality will be slightly more complicated, in principle the more widgets you make, they less they cost.
Translation doesn’t work like that. All the widgets manufactured in this example are the same: they come from the same design, created once and reused endlessly. But every text comes from a different context and is written and translated for different reasons, so the same approach can’t be used in all cases. Even the exact same sentence will be translated differently depending on where it appears, how it is used, and who it is being written for.
Every translation project has be to evaluated and approached individually. The basic cost involved is the time of skilled professionals, which doesn’t scale.