Yahoo is undoubtedly going through a rough time in terms of their share price, but what's being forgotten is past performance. Since the beginning of 2007 Yahoo's share price has averaged around $25 or so, with a low of $22.73 and a high of $33.63. The pre-bid low this year was $19.05, and was probably due to the fact that the majority of the industry was taking a beating, along with the rest of the economy.
When companies offer takeovers, they usually pay a premium in share price, however Microsoft took advantage of the state of the economy by offering a takeover at a particularly low point in share price, allowing them to offer an artificial premium that was - when compared to YHOO average share price - at only a $5 premium, or 20%.