What is the cost of a wastewater connection (holiday bungalow)?

The connection from the street to the transfer shaft is standard and you can inquire about the price from the supplier. The connection from the transfer shaft to the bungalow must be provided privately. That means connection costs + shaft costs + connection costs (supplier) can already amount to 5-10K€.

That means connection costs + shaft costs + connection costs (utility provider) can already amount to 5-10k€

I think so too: 1000€? for the building authority/utility provider, 6-7000€ for the earthworks... that would at least be realistic.

Thank you very much, I am glad to at least have an approximate number to be able to prepare for it. As it looks, the earthworks are probably the most expensive. Can the price be reduced by doing some of the work myself (digging the shaft) or am I being too naive?

Of course you can do that. We also laid our wastewater pipes on the property ourselves 20 years ago. But it's a lot of work if you don't rent a mini excavator.

Can you lower the price a bit by doing it yourself (digging the shaft) or am I too naive?

That’s possible. But it’s also very dangerous. The risk of soil in the shaft is not seen or understood by a layman. But you probably have earthworks contractors there with whom you can discuss a helping hand and small tasks. A mini excavator for half a day doesn’t cost the world either.

Can you reduce the price a bit by doing it yourself (digging the shaft) or am I being too naive?

Of course you can.