What is the value of anything?

I have noticed, near my workplace, several newsagents selling 2008 Diaries. They are advertising these as 20% off the marked price. It makes me wonder what is the value of a Diary at this time of the year? Consider. A diary is valid for 12 months. It may have some extra information, such as holiday dates, multiple year calendars etcetera, but primarily a diary is based on functionality over a calendar year. It is now June. Effectively the year is half over. Shouldn't the diaries be selling at 50% off? Is not half their usefulness now been lost? If I was buying a diary now, would it not be more cost effective and more sensible to buy a financial year diary and get a full 12 months use out of it, for only 25% more than the discounted Calendar year diary? What do these shop keepers think we are? Stupid?