Cost estimate for multi-foundation frost protection gravel 0/45 crushed grain

9,000 € for the additional thickness is too expensive.
He has to have all the equipment there anyway, possibly one more layer of compaction effort.
+ material costs of max. 3,000 €.

Meet at 4,500 and it's still well paid!

You should make your own assessment regarding removal and not rely on the company’s assumptions.
How much soil can be reused later? Is the soil usable, does it need to go to a landfill,...
That can quickly add an extra zero to the 5,000 €.
(Just the 85 m³ of additional soil are 5,000 € if you have to drive them to the landfill)

Do you already have the chemical soil analysis? Otherwise, transporting it might still hold some element of surprise.

Material costs vary regionally, since there isn’t a suitable gravel pit on every street corner. If the material has to be transported over longer distances, it gets expensive. €4500 would have unfortunately not been sufficient at all for the civil engineer in our case. Nevertheless, I also think the price is set high, but that might also have regional reasons.