Why are they so expensive? Is it just because they are new and their value will drop drastically within a few weeks? Or they are very hard to get and they will stay expensive for a long time?

hard to get

Really rare and really useful.

They are very hard to find and everyone wants one. As time passes, more of them will be found and less of them will be needed: now we had a spike in players to play the new char, but a lot of those players will leave and more and more players will already have one renewed, so expect a big drop in their value as time passes.
Don't think you are paying 10.000fg for a bone break and get 20.000 in the future, because this is not what is going to happen.

supply probably wont get much better since standard bots can't reliably find them

Overhyped charms tbh.

But they're really rare.

What you think improves your build more.

Upgrading lore into shacko or upgrading regular sunder into new rerolled sunder?

At some thousands per each one i bet my hand that they will fix that in no time.

Just upgrade your Lore into Shako and all will be good

Waste of resources, they wont even bother.

They are the most undervalued items in the game

Rare drops, hard to get with bots, and many variable stats. -10 or +15 is already way more worth than a latent even if the other stats arent the best. Some people sell them way too cheap imo, i bought sunders where people thought they were overpriced and sold them all with profit. Most people underestimate the value of these charms