Its the basic rule of selling - demand determines pricing. That is fair enough, but another thing that affects pricing of products is timing.
Take the case of Halloween costumes, have you ever got a bargain on costumes the weekend before halloween, its fascinating to see how normal costumes which would cost no more than $10, end up being sold for more than $40 a piece. It is all about timing.
This year i ended up buying mine and desi girl's costumes on craigslist, although the idea of wearing used costumes wasn't really sold on her, i did manage to save over $50 on two costumes. To avoid this same scenario for next year, i kept a close eye on costume prices after halloween, and rightly so - i ended up getting new costumes at over 60% discount for next year. So by being a little proactive (and at the risk of not being able to buy latest stuff for next year) i saved us some money as well as avoided wearing used costumes.
Now only if i could find a way to keep roses fresh for a year, because for some reason roses seem to become as rare as diamonds in the second week of February, and start costing almost as much as diamonds do :)