duese, any insurance contract for "buildings" cover will normally cover not just the initial payment, but usually the cost of not just rebuilding at current costs (which will be substantially higher in 200x than 198x*), but also quite often the cost of clearing the grounds and making the site usable again.
For example you buy a house in 1970 for 12k, that wouldn't cover the cost of rebuilding any house now
, so your insurance cover today would probably be for up to something like 500k to cover the cost of (in worst case) pulling down any remains after a major fire, clearing the site, then going through the planning process and getting it rebuild to current standards.
Without knowing how much construction costs have risen in the US (both materials and labour) it's very likely the sum paid out in the case of the WTC would only realistically cover rebuilding.
It's also worth noting that with some insurance policies for industrial/commercial cover they not only cover the physical costs, but also loss of earnings etc due to the cause of the claim (so in the case of something like WTC where you've got a large office/commercial building with rent from companies using it, the insurance may cover the loss of rental income).
*Although probably not in real terms (remembering inflation)
For example you buy a house in 1970 for 12k, that wouldn't cover the cost of rebuilding any house now

Without knowing how much construction costs have risen in the US (both materials and labour) it's very likely the sum paid out in the case of the WTC would only realistically cover rebuilding.
It's also worth noting that with some insurance policies for industrial/commercial cover they not only cover the physical costs, but also loss of earnings etc due to the cause of the claim (so in the case of something like WTC where you've got a large office/commercial building with rent from companies using it, the insurance may cover the loss of rental income).
*Although probably not in real terms (remembering inflation)