Both of these questions have complicated answers so sorry for the wall of text incoming.
The most difficult are the ones you don’t even know exist. There are at least 10 there that I have the only copy I’ve ever seen. Not saying others don’t exist, but when it’s the only copy you know of, more often it’s about getting that copy vs finding another.
The next step to that would be the very rare ones that it’s at least known to have multiple copies. For me personally, I try to keep the connections there for the copies I do know of in case they ever sell and do my best to keep it known I’m looking for them for when a new copy does show.
It always feels like when it rains, it pours. There was one style that I was hunting and hadn’t seen a sale in over 2 years. I posted again that I was looking for it, and 30 minutes later, in a completely different group, someone came out of nowhere with not only a copy of what I was asking for, but another Filler I had never seen before. Just that quick a new one exists.
As for value, I tend to bundle them into 3 tiers, the common styles, the rare styles, and the outliers. This isn’t a set in stone thing, just personally how I view them.
The “common” styles are just the ones you see most frequently ranging from $100 to a few hundred. Outside of some exceptions, this is basically any with 20 or more on the CGC pop report.
The “rare” styles are a step up from the commons. Something about them making them more limited. Small Detail or Change that is really only for the niche collectors. Most of these land in the 300-600 range in my experience price depending on a variety of factors.
The “outliers” can be an outlier for a few reasons. I’ll use the grayscale Pokeball for example, it’s probably closer to the “common” style in terms of rarity, but because it’s more than a blank white, it has more demand, even outside of strictly error collectors. Most of the holos follow this same logic. oooooo shiny has more demand than ooooo blank white. Then there are the ones that only 1 is known about. Which comes down to whatever someone will pay, and what the owner will sell for.
I base my offers and prices off of the best comparison I have for them, and any I want this premium I may need to add haha