don't waist your time on the Demonic ones...i'd go for Vabbi(easy to get to) and ancient and mergonites...so far the Vabbi's have been most productive for me.
But in all honesty if you only have 30k to spare..don't waist it on chest running..cuz more often then not you aren't going to find that rare elemental sword, gloom shield or dead sword..only when you have gold to spare is it a good idea.
Demonic chests were good to me, all before triple weekend. Including req 9 and 11 gloom shield, two +30 shield mods, req. 9 spear (nightmare?) and some other niceties.
From about 40k or so spent on keys I made around 200k.
Vabbi these days covers the cost of keys for me with around 50% profit.
Ancients barely covered the cost of keys even during triple weekend.
Margonites were a complete loss.
It's just my personal experience, in the end, it comes down to luck. Your first chest could be a low req elemental sword.
Unless you've got money to waste I'd wait until the next double/triple gold drop weekend, which most likely will be close to when Chapter 4 is released. Just spend it on a few keys now and then when you're bored.
Vabbi are the safe bet as far as covering your costs.
If your after good mods that is a nice way to find them.
If your truely after the high end skins you have to go for the top 2 chests, but be aware that the best drops are very rare and youll likely spend as much on keys looking for an elemental sword as you would just trying to buy one
I found that the Ancient Elonian chests, albeit holding the Dead and Ele swords, always disappoint in terms of mods and req. Problem is that in Vabbi I was only able to cover for 75% of what I spent on keys during my treasure hunter title run so I am thinking it is all based on luck.