We have one from Wildau and are very satisfied so far, but he has only helped us to compare / accept offers and we are still waiting for the building permit. For 10 appointments they charge about 4000€ gross. This includes inspections after important phases in the construction. But even with the offers, he told us what to pay attention to and corrected things, which has already helped a lot. Also always easy to reach by phone and takes time there as well.
For an all-round package, with warranty by the expert and complete takeover of the construction site and unlimited inspections / appointments 12,000€. But that was at the beginning of the year, don't know if the price has gone up. This is more necessary if you have no time to go to the construction site and/or there is no site manager at all, so they then act as the site managers.