I think a lot may depend on the catalog(s) you are using and/or whether you are talking about stamps, cancellations or, in some cases, perfins or precancels. So the "rarity factor" for one type of stamp or cancel may not necessarily hold true for another type.
Take for example, perfins, wherein this was previously quoted from another thread:
Quote:
US perfins are rated A (scarcest) to F (very common) with + adjustments for the BC&D's. General price levels have been going up. An A-rated can easily bring over 100$. DE&F's are fun but no cash value.
In reality, rarity factors are simply guidelines at best, not much different than quoting a value from the Scott catalog. The true value really comes down to what a buyer is willing to pay and a seller is willing to accept and that can vary significantly depending on any number of conditions.