The topic of landfill costs for excavation is the "shittiest" topic ever when it comes to civil engineering.
First and foremost, it depends on the quality of your excavation. Z0 contains no contamination. That means no organic material like roots either. Does anyone know soil without roots? I don’t!!!
For Z0, I had offers ranging from/to. The cheapest were between €18-23/m³ net excavation. So, digging out and disposing.
Z1.1 with roots and stones the size of a golf ball was between €38-45/m³ net.
Then there is contaminated soil with rubble, higher lead and pipapo concentration, etc. That’s somewhere beyond €100/m³.
If you want to build with a basement, anything above Z1.1 is unaffordable anyway. You might as well skip the basement. €20k just to get rid of soil is nonsense.
Or you find a civil engineer who can get rid of the soil of any quality under the table. That means sum x in cash.
Have you even done a soil analysis according to [LAGA], because apparently you can’t do anything with the Z-classification relevant for the landfill. Soil class 3-4 has only limited connection to rising landfill costs. The soil class only indicates how easy or difficult the substrate is to excavate.
And another tip: don’t let the landfill do the soil analysis at pickup. Do it yourself. It must not be older than 3-6 months. Otherwise, there will be a big surprise.