Indian cent varieties are extensive due to the nature of their creation and large mintages, and many collectors specialize in these alone. They can be difficult to price due to rarity/demand or lesser significance/demand. All are fascinating in their own right, their study adding to our knowledge of 19th Century minting procedures. The cataloging of new varieties is an ongoing process with discovery coins authenticated monthly. Even the best known 1873 DDO didn't make it to the Red Book until the late 1970s. Rick Snow has become the established authority for attribution, and his "S" numbers for variety types have revolutionized identification.
The 1873S1 and 1888/7S1 are the most sought after due to their rarity and unique status: the former (estimated population of 126) is by far the most visual double die obverse and the latter (est. pop. of 25) is the only true overdate of the series. The finest knowns of each appear on the following pages, and are the plate coins from Larry Steve and Kevin Flynn's The F.IND.ERS Report.
The simplest way to separate varieties from errors is to consider all die errors as varieties and all strike errors as errors. If the variation existed on the die itself before being transfered to the planchet, the resulting coin is a variety. This ensures more than one example with the same variety trait, making it "collectable". Errors, on the other hand, are typically one of a kind.