For Shadowpriests, PvP Power is worth roughly 1.2 PP (pseudopower), or 1.2 times the value of Intellect. So if you want to try to approximate its PvP value, this trinket says:
+ 2881 Spellpower with 33% uptime
Or 960 average spellpower bonus - but remember that you are also using your on-use trinket during the time you are using all your burst damage, so it's functionally way higher than just averaging it out over the length of its cooldown.
So the PP value of this trinket is worth about 3000 even if you just use it on cooldown and Don't sync it with your burst, if you sync it with Devouring Plagues and Mindbender or Shadowfiend or Insanity or DI procs or Power Infusion - it obviously leaps way way up from there.
Then it also decreases the damage you take by 1.2%, except - when you already have 65% damage reduction (full pvp gear) -1.2% isn't 1.2%, it's 1.2 / 35 = 3.43% damage reduction, even with the diminishing return on resilience it's still going to be worth about 2.5-3% damage reduction on top of a like ~3000 PP value, that is only 3000 because it assumes your using it on cooldown, including when your CC'd or not on combat or Line of Sighting or the like: not syncing it with burst.
Lastly, it's also important to note that when comparing it to PvP trinkets - PvE PP values are calculated assuming PvE gear levels and stats, and full raid buffs - so even our BiS 541 item level trinket would look way less valuable if you were simming it around pvp stats that are probably close to half the spellpower / haste / crit / mastery levels of BiS PvE spriests: and all the synergies that results in.
As far as PvP is concerned, if it says "PvP power" and/or "PvP Resilience" on it, it's better than any PvE item.