2008 US State Quarter Sets (S Mint - Proof)

From what I can see by glancing at the Red Book, it seems that the 2008 proof quarters had a mintage of about ~2,078,000 while most of the other years had between ~2,374,000 to ~3,713,000.

So, they have at least around 300,000 less minted than the next most scarce date. Other than that, the premium must come from the fact that that was the year of the Oklahoma quarter. :P

Bonner is correct, the cost of the 2008S clad proof set is dictated by its lowest mintage of the 50 State Quarter clad proof and by the fact it is also the last year of the series. It's also been heavily speculated on by the HSNs and mass marketeers of coin collectibles, promoting its "rarity." its neither scarce nor rare relative to the proof sets before and after it. As more sets come back into the market, the price will eventually come back down. At one point in 2010, the set was more than $20 over the cost of the '08 silver proof set. You will find the same scenario for the '08 mint set, heavily bought out by promoters, accounting for the excessively high premium for them.