You have to remember that D&D is an abstraction of "reality". Just like HP are a basic abstraction metric used to track ... damage, fatigue, loss of morale from near-misses, etc, etc... GP used to track spell-component value is also a basic abstraction.
A 100GP pearl is just saying that the pearl meets "time and quality" expectations for the spell. In the game's reality, maybe these pearls that can be used for an identify spell were washed in the tears of a dragon and soaked in the rays of a full moon for a year. Who knows. All we know is that they're 100GP, which means there's something pretty special about them.
Also, while we can try to apply real-world concepts like economic supply and demand (pearls in one town may be 90GP while in another town they're 110GP, or low-quality vs. high-quality has a range of +/- 50GP or something...) ... again, you can BS by saying that maybe someone in one town found a way to prep pearls for the spell a certain way, but a person in another town found another way to prep them ... and while both methods may be different.. they both end up creating 100GP pearls that let the spell work.
If you think about it, spells are a lot like math in that they have been researched by magicians and they found different ways to get the same result. Just like 1+2 = 3 and 1+1+1 = 3, some magician may have found prepping pearls a certain way worked for the Identify spell.. but you have to then use them a very specific way. Meanwhile a different magician also studying pearls found another way to prepare them and use in a different fashion .. but, again, the end-result was the Identify spell working.
This is why you shouldn't dissect the abstraction of D&D too much, b/c once you start questioning it or trying to shoe-horn real-world concepts into it to analyze it in more detail, you can start to poke holes in it if you want. But, since the rules are an abstraction, you can then come up with reasons why it SHOULD be the way it is.
EG: people have argued over the concept of HP for a long time.. is it physical damage, is it fatigue, and if it's real damage it should take a long time to heal, but if it's fatigue it should heal fast.. etc, etc. The bottomline is.. don't over-analyze it. It's just a game mechanic used to regulate play.
Same with 100GP pearls needed for Identify spell. It's just an abstract game concept used to keep munchkin players from going "well, I'm gonna start a pearl farm, and it's real cheap to raise them, I calculate about 5GP/pearl.. and I'll sell them for 100GP, so that's a 95GP profit/pearl!" Um.. no. We have no idea what goes into prepping the pearls to make them useable in the Identify spell. (Some spells will say how the reagents are used, eg: oil is rubbed on something to do such-n-such). But, as I said earlier, we have no clue if a spell reagent had to go through certain periods of consecration, ritual blessing, etc, etc. All of this is abstraction. The GM can fill in the gaps to add flair via story-telling, but all of it is just abstraction to justify pearls costing 100GP being the ones useable for the spell.
I've played in some games where the GM takes this concept and goes a step further by letting players buy up skill in "reagent preparation". Because, let's face it, stockpiling an MU's reagents can get expensive. So, characters that had enough skill in this could start to find and prep reagents themselves as they found them while exploring.. saving the cost of having to go to "bob's discount reagents emporium". Because, after all, SOMEONE had to figure out how to prep the reagents in the first place, so it makes sense that it's a skill that perhaps magic-users can acquire and develop. But, all this did was work as part of the abstraction. An MU that found some pearls in water that would normally go for 50GP each knows the exact method of preparation to use them for Identify spell... what that method is.. who knows. All we know is that the time and effort they spend in prepping them adds another 50GP value to the pearls and makes them "100GP pearls that can be used in the Idenitfy spell". And, with that, the MU could then sell them as reagents for magic instead of just normal pearls.
Beacuse you have to figure in the world of D&D.. there's probably shady back-alley reagent dealers trying to hock shoddy reagents onto MU's. How do you know the pearls you're buying can be used for the Identify spell? You don't unless a) you have skill in identifying properly prepared reagents, b) you use the reagent and .. oops, it fizzles.
To solve all this, I've played in some groups where they just ignore reagents save for really high-level spells. The fact that MU's are limited to using certain number of spells per day is often restriction enough. If an MU wants to toss fireballs, then they can up to their max spell cap for that level. But, having to waste time seeing if they're pack-ratting all the reagents to cast it.. that just adds overhead to game play.. it slows things down. And you end up with players having to write out huge inventory lists to manage all that junk. It's boring. Those games also ignored food and water, because they wanted to focus on heroism, not "did we pack enough rations to last the entire dungeon? No... ugh.. we have to hike back to town." That's great in a CRPG, but it slows table-top down.