One simply takes the construction price index. The point of construction start (or whatever is defined in the contract) and compares it with construction end. That makes it zero complex.
No. If things go the way the thread starter described, then the developer wants to use their actual purchase prices for individual orders as a basis. Bricks, stones, mortar, cement, wood, construction steel, windows, etc.
This is not only very difficult for the client to verify, but in the current situation it is also an extremely high risk. I already said it, if the situation further escalates, we have a harsh winter and the building materials industry has to shut down plants because there isn’t enough gas available... then prices for bricks, cement, stones will skyrocket to levels we can’t even imagine right now...
Forget it, that is my advice. This will almost certainly escalate into a dispute... and with my general contractor I have always very reluctantly argued, because he always holds the upper hand.