Average Click-Through-Rate for web ads is about 0.5%.

That translates to about 20 million ad clicks per month.

Average Cost-Per-Click is about 0.5 USD, meaning Yahoo earns about 10 Million dollars from ad clicks.

Lets triple it, considering exclusive deals, ad deals, etc. So thats 30 M USD per month, or about 360 M USD per year.

Still is it worth about 5 B USD?

Also note that this is current rates. Internet is a finicky business where if you drop off the radar of the most active users, you will lose revenue very quick.

So the risk adjustment factors for companies like Yahoo will likely be very high.