I have found this site by the US Currency Education Program most useful. The main menu might be more accessible as it provides different languages as well although @rsirian1 has explained things well. 😀
About everything that made me very upset and I threw everything on the ground emotionally. Who will explain it? How can a UNC bill be valid - rated on average that it can be bought for $1.90.
Ivan, I don't take the pricing tables very seriously on Numista since they're based on a lot data which may (or more likely may not) reflect what a note actually costs (or can be sold for). Obviously, $1.90 doesn't make sense for a $2.00 note suggesting a problem with the software that calculates these numbers.
What likely complicates things, is that many users have secured the note for FV (or Face Value & paid $2.00 for a note which may be tough/hard to source). The same note may cost other collectors 150% - 200% FV (& perhaps more, depending on many factors). I would suspect that the newer the note (& the lower the denomination) the more unreliable the tables will be. For older notes (& higher denominations -which typically cost collectors a significant premium) the data from the tables would likely be much more reliable. For all catalogues, collectors must remember that the tables are made up of 'ballpark" figures, approximations, just like greensheet provides dealers with a starting point (a $X figure that could be negotiated).
Hope that helps!