Some factors determining US proof set values include:
- The grading service – many US proof set coins are graded by rare coin grading services like ANACS, NGC, and PCGS.
- The grade of the coins – Let’s face it, quality can matter. Proof coins in the very highest of grades could be valued significantly higher.
- The year of issue – Modern US proof sets have been minted since 1936. Generally speaking, US proof set values are higher the closer they are to 1936. However, throughout the series there are special instances of various sets going for hundreds or even thousands of dollars more than normal.
No one else can help you better ferret out those valuable coins than us.
For those interested, here is a link to a deeper dive into Proof Sets, Mint Sets, and Commemorative Sets.