I have not yet fully understood whether the 21.4% increase actually applies to the entire construction sum. If it is the total construction sum, then with a 0.6% adjustment margin per month, at least 36 months of construction delay would have had to pass if you simply add it up. Using the "compound interest" method, you still come to at least 32 months. I remember that the matter has been dragging on with you, but that long? Your supplier will have to prove the price increase - which he might be able to manage with 34 months. I would listen to everything calmly tomorrow, have it explained in writing, and then seek advice. Do not make any commitments, look at the samples and do not make a binding decision. You can make your own notes.