Costs for completion and warranty bonds?

The interest rate indication always refers to the amount of the guarantee. In the case of a warranty guarantee, this is logically in the amount of the warranty. (usually 5% of the trade/invoice)

So, based on your invoice: 240,000 euros, of which 5% is 12,000 euros (guarantee), the costs amount to 30 to 360 euros per year, resulting in costs of 150 to 1800 euros over 5 years of warranty, plus any possible notarization fees. As you can see, the range is quite large, but these are only approximate values. It is unlikely to be cheaper, but it should not be much more expensive either.

Oh, the warranty only covers 5% of the trade? That sounds pretty little... And what if after 2 years some huge damage occurs, just as an example, for EUR 30,000?

Then you have the possibilities given to you by the Baugesetzbuch or, if applicable, other contractual agreements.
A warranty bond is not a comprehensive insurance.

Just imagine the costs a BU/BT/GU/GÜ/HW would incur for such a bond if it amounted to over 30,000 TEUR? Then you would lose 4,500 euros just because of that, which a good provider would have already included in the offer price and which you ultimately pay as a customer.