This is all due to the way collectors collect. For example, I have dansco albums for pennies, nickels, dimes and quarters and halves, but I only include proofs in the pennies, nickels and dimes. So I order one proof set and break it up for the pennies through dimes and sell off the quarters, halves and dollars. This math makes sense since the clad set might cost $27, but if I bought those three coins separately I might spend $20, so I can usually sell the extra quarters, halves and dollars and get more than the $7 difference and I'm ahead. I used to do the same with a silver proof set, until the price of that set became insane, so now rather than buying a silver set just for the dime, I just buy the dime separately for $8 or $9 on ebay, which is higher than it might warrant from a set price standpoint, but for me, $9 for the one coin I need is better than $83 for the one coin I need and 8 or 9 that I don't need, and I wouldn't need to deal with the hassle of selling the extras.