Proof notes are highly sought after among collectors. Compared with unissued remainder notes or issued notes, their population estimates are generally much lower. Remainder notes are sometimes common as dozens of sheets may exist for certain banks, The Citizens Bank of Louisiana is one such example where plenty of remainders exist. Issued notes can be worth more than a remainder note if they are less common than the remainders. However, issued notes typically can be found in various grades where some of them may be so worn out that they are inexpensive despite the rarity of a note.
Since roughly 8 or so proof notes were made for each variety and the American Banknote Company sold their proof note archives in 1990, most of them are in pristine condition and thus appeal to a plethora of collectors from pre-civil war buffs, to numismatists, to art collectors, not to mention those who purchase particular notes based on theme or sentiment. After all, anyone watching the Antiques Roadshow has heard them mention numerous times: "Condition Condition Condition".
A little over 10 years ago many proofs sold in the $100 to $300 range but now tend to sell in the $1200 range, although some can still be found for several hundred dollars (See: Reasons To Collect Obsolete Currency Instead Of Coins). Recently however, some proofs have sold for tens of thousands, as attested by the recent sale by Heritage of notes from the Roger Durand Santa Clause Collection. A couple notes from this collection included a $5 proof from the Saint Nicholas Bank of New York, NY and a $2 proof from the Knickerbocker Bank of New York, New York. These notes sold for over $10,000 and $20,000 respectively.
What made these notes go for a lot is the fact that they are extremely rare with the $5 note being only one of 2 known and the $2 proof being one of 3 known (Source: Heritage). Moreover, certified banknotes tend to fetch a lot more than non certified ones as their conditions have become established by a 3rd party grading service. Santa Claus themed antiques are generally popular among collectors and these two notes are much earlier than what is generally available to collectors.
Therefore proof notes which are graded, extremely rare, excellent condition, and which have unusual and rare vignettes on them are exceptional investments.