Slight frame-change: don't worry about the price.
It sounds like you're creating a custom variant of a monster, in effect: you've got a bugbear with a Vorpal blade or a wizard with goggles of True Seeing or an Invisible Stalker with a bracelet of Silence. In any case, the item is "soul-bound" to the wielder and is destroyed upon its death. That sounds a lot like slapping a poisonous stinger onto a dragon's tail slap.
The specifics will vary, but this GM's advice is to look at the Adding Special Abilities section of improving monsters:
You can add any sort of spell-like, supernatural, or extraordinary ability to a creature. As with a class level, you should determine how much, or how little, this ability adds to the creature’s existing repertoire. A suite of abilities that work together should be treated as a single modifier for this purpose. If the ability (or combination of abilities) significantly increases the monster’s combat effectiveness, increase its CR by 2. Minor abilities increase the creature’s CR by 1, and truly trivial abilities may not increase CR at all. If the special abilities a monster gains are not tied to a class or Hit Die increase, this CR increase stacks.
A significant special attack is one that stands a good chance of incapacitating or crippling a character in one round. A significant special quality is one that seriously diminishes the monster’s vulnerability to common attacks. Do not add this factor twice if a monster has both special attacks and special qualities.
Make sure to "scale" your evaluation of these abilities by the monster’s current CR.
Treat the effect of using the item as an SP, SU, or EX ability (keep in mind the fact that SP and SU are vulnerable to anti-magic effects, like magic items are).
That bugbear's so good with their sword that, as an EX, they will decapitate their victim on a crit (like the Razor Boar's tusks).
The wizard has made a pact with a demon which has granted them the supernatural ability to sense the presence of any living creature within line of effect, piercing all illusions.
The Invisible Stalker's been infused by the demielemental plane of silence ('cause, why not), and sound within 30 feet is immediately absorbed.
Which, this GM would peg all three of those abilities as "significantly increasing the monster's combat effectiveness".