The standard measure for such things is the Scoville Heat Unit, or SHU, named after Wilbur Lincoln Scoville, a chemist who in 1912 developed a methodofassessing the heat given off by capsaicin, the active ingredient in chili peppers.
MacDonald ran his numbers using equivalent GDP ratio, which he says is a better method than inflation-adjusting when it comes to assessing relative historical impacts of megaprojects like telescopes or churches.