Given this, because VIP chests are stated as x100 chance and Gem chests are stated as x10 the rate of glory key drops, the likelihood for gem chests would be 1/1000, and the average gem cost from event chests would be 12.5k gems. So very much worse.
Every mythic I obtained seemed to be within the 1/5000 glory chest 1/500 gem 1/50 VIP chest range, except Death, where I spent 7k glory keys, 100 VIP keys, and over 150 gem keys during his exclusive period and did not get him. Though a lot of estimation still has to be done, since the number of total mythics dropped is so very small.
Incidentally, a 1/10000 on glory chest rate would make the average cost of a mythic during its exclusive period higher than the total amount of combined gems (toward VIP chests) gem keys, and glory keys available from guild tasks during a month (about 9384 glory key equivalent when hunting for mythics, if you are using all gems on VIP chests in 50 packs), but this is not considering guild chests or personal gems obtained via multi-tributes. If you spent every gem and event key obtained from your guild toward getting it during a kingdom event, it would take 12 weeks for you to obtain 1000 event keys equivalent. Without spending any raw gems (since you'd more likely save these for VIP chests if you wanted max mythic chance, going forward) you are looking at a 24 week period. And thats not considering you'd probably want to be spending those event keys on new legendaries as the pop up anyway.
Theres also the small matter that spending 1000 event keys on a kingdom with one legendary could not possibly even give you progress toward ascensions anymore after the first 250-300 or so, because you'll have the legendary to Mythic by then, and every other troop available from an event key to mythic three times over.
No matter which way you look at it, unless there is dev confirmation on at least the relative drop rate for mythics on event keys versus other key types, it is not worth it to try and focus down a mythic drop from a kingdom event. Now, if needed a couple Crimson Bats to mythic it and you wanted to go for that, and you happened to get Drakk to drop, you'd be extremely lucky, but this is the only situation where I can think of it being "worth" it. But targeting Drakk is the event prohibitively expensive, whether you are looking at a 1/5000 or 1/10000 base glory key drop rate, or anywhere in between.