(Post edited for brevity, clarity, and a bit of zip and zazz!) HEY, MARK
#2 Break Open (Onslaught) Why is it bad
Rather than writing everything out, using multipliers, and doing all of the stuff we do for the article, you can simply add up the total value of all the mythics in the set (in this case, roughly $115), divide by the total number of mythics in the set (generally 15 but sometimes slightly different Throne of Eldraine has 15, which would give us $115.05 / 15 = $7.67) to get the average value of a mythic rare, and then finally multiply by the number of mythics you'll get from an average box (here's a cheat sheet: the average box as 4.5 mythics, 31.5 rares, 108 uncommons, and 360 commons)
Evolving Skies in particular has been stuffed into various tins and such lately, so there are plenty of sources