The 1864 Seated Liberty Silver Dollar is very scarce in Proof condition. The mintage of regular issue coins during this Civil War year was only 30,700, so collectors put a lot of price pressure on this date because a Proof 1864 Dollar is often easier (and less expensive) to find than is a nice Mint State example.
The survival rate of Proofs of this date seems fairly high or, at least, those that survive are in nice condition. The PCGS Condition Census is filled with PR65 or better examples, and the best is a single PCGS PR67.
Cameo Proofs represent about 30% of the population at present, but that percentage is expected to increase in time as older submissions are reevaluated (PCGS did not recognize Cameo and Deep Cameo designations until years after they began certifying coins). Deep Cameo 1864 Silver Dollars are non-existent at present.