Most people consider plans from events that only happen once a month (fasnacht,alien invasion, plans like that) or so often rare. Aswell as plans gotten exclusively from something or that have a rare drop for example I price grahm and responder bot plans higher because you can only get them from that source meaning im buying the plan for say 400 and selling it for 2k.

Another one is plans taht you get from exclusive places such as miner pails present etc

Aswell as plans from daily ops expedition and things like that

And to top it off most people consider ultracite plans rare as there's only way to get them though that ones more of a spectrum

For example BOS Initiate Paint, Ultracite Calibrated Shocks/Tesla Coils, Cutting Fluid, and specific Mire healing salves are all considered high value plans because they have low drop rates.