TheHutchCom The 100 gem stories are all low energy to complete the main quest line. They aren't segmented so the main story, treasure and achievements were all set to be easy enough for players in Egypt. People didn't mention it because most (all?) of the stories on special are like that for the main quest line. Many are low energy overall (the ones from 2014 and earlier) - say less than 400 K energy to clear all the mines.
At some point, they started putting "Hard" areas in the main story line maps to have a harder area for players in later regions. The downside of that is that Egypt players have to stretch pretty hard to get the 10x map clear bonus for the maps. Later (sometime in 2015), they changed to putting the Hard areas in separate Challenge maps.
Christmas 2015 is the highest energy requirement overall. The main story line is cheap, but some of the challenge mines require a lot of energy. Clearing all the Challenge maps takes almost 4 million energy, though of course one can get away with less energy just going for the reward areas and scattered bells on the harder challenge maps.
Edited to add: The events page on the Wiki now has a spreadsheet summarizing the 100 gem events ordered by cost in gems. Scroll down below the event page links to see it.
Their gem cost calculation doesn't agree with some of the ones tcpolymath posted above. They say that Independence day has 95 gems so net is -5 gems and Wild West is much lower. I checked Wild West. It looks like the Wiki gem count of 59 only counted the gems from one of the 3 achievements. With all 3 achievements, it provides 104 gems as tcpolymath said above.
The wiki points out that Christmas vouchers can be used for green rings, but I think the Christmas 2014 exchange for jewels is a better use.