How do I estimate the cost of one week of delay?
Add up every cost that runs by the week once the planned completion date passes: rent on the new premises you cannot use, holding-over charges on the old premises, lost trading revenue, interest on project financing, extended consultant fees, temporary storage, and staff time. Use your own lease and budget figures. The resulting weekly figure is the number to compare against any difference in contract price.
From the RRD Field Guide, checked against the sources cited in Cheapest Quote vs Delay Risk: The Economics of Choosing a Contractor in Singapore. General information, not advice for your project.
The fuller picture
The cheapest quote is only cheaper if the job finishes on time and on scope. Overrun research shows delays are common and fat-tailed, so a small price saving can be wiped out by a few weeks of rent, lost revenue or holding-over. Compare quotes on expected total cost, including probability-weighted delay cost and the worst plausible outcome, not the tender price alone.