Warning: Math ahoy!

Crit Chance:

Both Physical Crit (PCr) and Spell Crit rating (SCr) are converted from their primary stats at a rate of 50%; so 80 STR/DEX or INT/WIS becomes 40 Physical Crit or Spell Crit rating. PCr and SCr are also converted to Crit Chance at a rate of .008% (1/125th chance per Rating); so 1000 Spell Crit becomes 8% chance to crit.

Crit Power:

Bonus damage on a critical strike is calculated via Crit Power (CrP) and starts at +50% damage without any rating (according to Patch Notes for 1.11; haven't found anything newer than that to clarify current state of crit damage), and increases by +1% per 28 CrP up to a maximum of +100%, which is 1400 CrP.

Fully upgraded T3 crit power gear (only armor pieces; and not best in slot but obviously still good gear) comes in at +748 Crit Power. T2/T3 equivalent weapons with CrP have 85-100 CrP on them, so make that a total of +848 CrP. That's +30% Crit Damage from rating (total Crit Damage is +80%).

Raid buffs give you +103 rating, bringing you up to 951 CrP (+33%; total +83%). The Flamesource proc will temporarily bring you up to +37%, totaling at +87% Crit Damage.

That is a whopping potential 4.8% DPS increase from CrP (87% versus 83%)... if you have 100% crit chance, which is impossible because of the hard cap of 60% Crit Chance from rating (though bonuses in your soul tree can bring it past that). The soft cap is 45% Crit Chance, which is pretty hard to break without buffs out of the wazoo (or being a rogue). If you are exactly at the soft cap, that makes the potential maximum DPS increase ~2.168%. That's assuming you crit every single hit in a fight. You won't. When you don't crit, CrP isn't worth anything. Crit Power, while useful for making crits multiplicatively more effective, is worth nothing in short and medium encounters without a guaranteed crit mechanic (or extremely high base crit chance). In raids, it does have value. But 103 CrP is not worth the 3000 - 4000 plat that a Charred Flamesource will run you.IF there was a tertiary upgrade from what Flamesource et al build intoto boost its stats, then it could be worth something. But as it is, you're paying 3500p for a wholly negligible boost in damage output. The increase is actually even less than what I've calculated here because I've excluded bonuses from accessories and Planar Focii/essences (obviously your mileage will vary since most people will have different amounts of CrP on them due to different equipment).

This post might seem a bit complainy over the price, but I don't mean for it to be. I just wanted to do mathcrafting for myself to see why it was so desired, and as it is, I don't see why it's so expensive other than rarity and the potential for getting an item shard first (though that is long since gone). Maybe it's purely an e-peen item. (Trion, I just got a great idea for a store item that enlarges your character's staff... Pm me for details )

tl;dr - The more Crit Power you have, the less of an effective DPS/HPS increase getting more will have. Flamesource is worth getting if you have everything else as BiS and you want to top yourself off. It's not really worth it in any other case, as you can buy equipment upgrades for more stats at a lower price.

If you stayed long enough to read all this, thanks!

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