Are you referring to Microsoft's 2008 offer to acquire Yahoo? That offer amounted to $31 a share, YHOO today is trading at $43-44 a share.
It is best to look at market cap to determine value - Microsoft's 2008 offer was $44.6B; Yahoo's current market cap is $41.65B.
The discrepancy from then to now comes from the number of shares outstanding. There are fewer outstanding YHOO shares today than there were in 2008.
What % of that is their core business that MS offered to buy?