Details appeared what Microsoft offered in the last weeks before Yahoo! decided to end talks by entering an advertisement deal with Google.

Apparently Microsoft was offering $9 billion in cash and $1 billion per year in additional operating profit.
Reuters reports that in an e-mail to employees, Microsoft Platforms and Services Division President Kevin Johnson said it had offered $8 billion for a 16% stake in Yahoo! and $1 billion to buy Yahoo's search business and assume its operations.
The proposal also included a revenue-sharing partnership that would have delivered $1 billion a year in additional operating income to Yahoo!.

For Yahoo the Google deal is of course less of a problem as it is non-exclusive. The Microsoft deal would have meant more money but loss of control. On the other hand, according to Microsoft over 90% of the paid search advertising market is now in Google's hands.

More details on Reuters.