I went through the theory of elixirs / flask recently, here are the results (forgive stating the obvious, it was originally intended for non-druids as an overview).
Assumption: it is possible with your best gear to be at crit immunity border when using elixir of ironskin (so the resilience is not wasted).
Elixir of Major Agility (35 agi, 20 crit rating) ~= 2.4% dodge, 0.9% crit
Elixir of Ironskin (30 resilience) = -0.75% to be critted. Comparing it to a Major fortitude elixir (250hp), which equals about 16 stamina, Ironskin is 3x better when it comes to itembudget. It enables to sub some suboptimal pieces of equipment that has defense on it for equipment without defense, same goes for enchants and gems.
Itempoints of elixirs: 85
Itempoints of their defensive part: 65
Alternative: Flask of Fortification (500hp, 10 defense). 500 hp = cca 32 stamina. Itempoints of flask: cca 32.
Meaning elixirs have approximately twice as many points in defense then flasks and above that boost threat as well.
An example of the difference according to a simulator. Total mitigation/effective health (physical dmg) raid buffed. This is set to work in a fight like Brutallus.
Elixirs + stam food: 84.2% / 86 300 (mitigation/effective health*)
Flask + stam food: 83.5% / 86 140 (regemmed 15 stam --> 10 def)
unbuffed: 83% / 83100 (regemmed 15 stam --> 10 def)
*effective health = how much damage unmitigated you survive without a heal = armor + hp, total mitigation is armor + dodge
Said differently, out of 100k dmg taken (unmitigated), you'll receive:
unbuffed - 17000
Flask - 16500 (cca 4% difference against unbuffed)
elixirs - 15800 (cca 7% difference against unbuffed)
Gear used in this simulation:
6/8 T6 (wrists) with 15 resilience on chest, 12 defense on wrists, glyph of the gladiator.
[Brooch of Deftness]
[Slikk's Cloak of Placation]
[Belt of Natural Power]
[Treads of the Den Mother]
[Ring of the Stalwart Protector]
[Violet Signet of the Great Protector]
[Commendation of Kael'thas]
[Badge of Tenacity]
[Wildfury Greatstaff]
[Idol of Terror]
Since this is my Brutallus tank gear, i had to replace the belt with cat belt in Rawr otherwise the changes in gear allowed by elixir of ironskin (e.g. armor on the badge ring) wouldnt be reflected properly in the overall mitigation.
About the costs: i think at this level the costs of the elixirs are not a problem. On progress content, you want the best you can get. On farm content, you dont die so the costs are minimal. I havent used a flask for a very long time now.