I initially wanted an 1851-O for my type set as well but it turned out to be a difficult task. I eventually settled on an 1855 that I lucked into, a friend purchased a 1851-62 VF/EF 3CS collection so I got a very nice VF30 for VF ask money. The 1851-O was VF20 but he kept it for himself
Three years later, I still have not found an equivalent 1855 or even a similar 1851-O.
Other thank those two coins, I think anything after 1862 tends to be undervalued compared to the true rarity of the business strikes. It is easier to find proofs in almost all cases. Unfortunately the whole series tends to be ignored except for type purposes so that would probably be the main reason for any perception of being undervalued.