Real question, is this a thing where you should be penny wise and pound foolish, or will too much risk doom the company. Lets say the final cost is $25 billion and not $20 billion or it is $30 billion and not $25 or $20, well if you are 1 year late, or 3 years, or if you do not have the right amount of capacity you may doom your marketshare of leading node supplies forever.
Yet if you overspend you may just be wasting money, and if this was some other company that did not have all these already made fabs you may be risking bankruptcy. And even with all these other fabs already made you just have to look at the RAM or Flash memory markets to see many companies go bankrupt turning sand into dollars / gold / stock money.