We are possibly talking about additional costs amounting to approximately €300-500!? We ourselves have negotiated a lot, calculated, planned, but still, there are small additional costs everywhere that simply have to be factored in. I would generally plan for 5-10% additional costs from the start.
This way, you don’t have to renegotiate harshly with the individual trades. Saving €400 and ending up with a disgruntled craftsman is not worth it to me. We didn’t haggle down to the last euro (although we negotiated very good prices), the craftsmen were constantly supplied with fresh coffee, water, and cake, and in return, a few things were repeatedly done quickly or slipped in additionally by the craftsmen present outside of the contract.