Who is familiar with pile foundation?

Hello everyone,
our house has to be built on piles (depth 3.5-4m). No problem, we knew that in advance. The seller of the prefab house company said back then when buying the house that 2000-3000€ is a lot, because the excavator would be on site anyway and for that money you would get a lot of concrete. Now the foundation slab builder (construction company = cooperation partner) says that this is definitely not possible at that price. We have now requested a concrete price because recently 7000-10000€ was being discussed. We do not know what is justified for a foundation slab size of 78sqm + 50sqm garage. In addition, he wants to reinforce the foundation slab a bit but by how much? We also do not know if 9 or 12 piles with garage are sufficient, as this quantity is planned for our timber frame house.
Who can give us a tip or say which costs would be reasonable.
Best regards, sonne

We were offered a plot of land where we would have had to do that as well. Cost per pile as the first offer was nearly €800. Altogether, the costs would have added up to almost €30,000. However, we did not inquire further and withdrew from the offer.

H E L P the absolute C A T A S T R O P H E

Hello everyone,
our house has to be built on piles (depth 3.5-4m). No problem, we knew that beforehand. The seller from the prefab house company said back when we bought the house that with €2000-3000 you could do a lot, because the excavator would be on site anyway and for this amount you would get a lot of concrete. Now the slab foundation contractor (construction company = cooperation partner) says that this is definitely not possible at this price. We have now requested a concrete price, because lately figures between €7000-10000 were mentioned. We don’t know what is justified for a slab size of 78 sqm + 50 sqm garage. In addition, he wants to reinforce the slab a bit but by how much? We also don’t know if 9 or 12 piles including the garage are sufficient, since this quantity is planned for our timber frame house.
Who can give us a tip or say which costs would be reasonable?
Cheers, sonne

We agreed with the slab foundation contractor on €10000 at a planned depth of 4m.
A few more details about the building site: a company used to be on this site years ago, the city bought these old buildings, demolished them and developed building plots.
All expert reports were made by the city, showing that all harmful substances found in the soil were removed and disposed of. Therefore, there was no reason for concern in this regard since the environmental authority also conducted inspections and granted approval. We knew that the soil was moved and filled, which is why piling was planned.
Now to the catastrophe. When digging the first pile, even after 5m no natural soil was found and here comes the catch: plenty of contaminated earth, which we suspect is contaminated with heating oil.
The city, the police, the district office, and our lawyer were notified immediately. The construction site is now stopped, analyses and investigations are ongoing. After just one day, liquid had already accumulated at the bottom of the pit, estimated at 40-50 liters. The surrounding area smells strongly. The neighbors who have already built their houses are now also very unsettled about what is going on under their houses.
Who is now responsible for ensuring that appropriate measures are taken and above all, who is liable and will cover the costs of the soil remediation?