Well, if the clerk told me that they could only be sold as a set, so that was the price per set, AND I took it home and used it for a few years, I certainly would expect the store to honor the contract, even if the clerk had made a mistake. I would expect the store to honor the sale once the item had left the premises.
Something similar did happen to me, and the store had to honor the original sale. I asked the clerk if he was sure the price was correct and he assured me that it was the correct price, so I put a deposit down and put it on lay away for Christmas.
I was contacted about a week later, and told that the clerk had made a mistake. Long story short, after contacting the AG's office, I was told that the store had to honor the original price, but did not have to allow me to defer payment, so I went in, paid in full, and took it home. I felt no moral obligation to rectify the mistake, as I only purchased the item because it was being sold at what I considered a bargain price - which I think was stratman's point. He would not have paid the asking price but for the CMs telling him it was good refills forever - which made it a bargain.