It is worth 100gp. Don't overthink it.
Although in reality taking a valuable diamond and turning it into powder would likely reduce its total value, D&D/Pathfinder does not model prices of luxury items in that much detail. Instead the spell component cost is supposed to be a mechanical transaction that limits number of castings of a spell. You input 100gp of funds (plus in the narrative it is reasonable to obtain the precise component you need), you can cast the spell.
As an aside: Would a craftsman be able to turn 100gp diamond into more than 100gp of diamond dust? No, the starting material and end material have to make sense in order for the craftsman's work to have added value as per the rules.
The Pathfinder craft rules are at best guidelines to enable characters to attempt to fix and make things. They are left open-ended about what is allowed or not, and include DM assessments of item complexity for instance. I would personally say that creating diamond dust from a diamond is not at all complex, nothing relevant to any Craft is being "made" and therefore it does not require a Craft check. The idea that it might take many weeks of effort for a skilled jeweller to turn a diamond into dust seems very wrong too.
If completely arbitrary transformations were allowed to gain value in combination with Craft and/or Fabricate, you could start with e.g. 10gp worth of raw gold, convert that into powder, convert the powder into jewellery, convert that back into something else (maybe just powder again, after all melting and forming gold is typical real-world crafting process) etc, until you have 10,000gp or more in funds.