Ron Guth: The Proof 1867 Seated Liberty Silver Dollar is a scarce coin with a total mintage of only 625 Proofs. It's not the lowest figure for the With Motto series (that distinction belongs to the 1868, 1869, and 1873 -- all with 600 Proofs each). Nor does the 1867 have the highest Proof mintage (that honor is reserved for the 1870 with 1,000 Proofs). All of the dates in this series are priced similarly despite the differences in mintages, perhaps because the surviving populations of each date are nearly identical according to the PCGS Population Report. Thus, the 1867 Seated Dollar is considered a type coin in Proof condition, though the buyer gets a slightly better mintage.
Over 10% of the Proofs certified by PCGS thus far (July 2012) are Cameo Proofs. Deep Cameo versions are rare, and they are usually impaired by hairlines from old cleanings. The best Proof 1867 Seated Dollars certified by PCGS include a single PR66+, four PR65CAMS, and a single PR66DCAM.
When selecting a Proof 1867 Silver Dollar, look for a strong strike, surfaces that are as free as possible of hairlines and/or contact marks, strong cameo contrast (if it exists), and appealing, natural color.