Online businesses are valued like many other businesses -- based on a multiple of key metrics.
The major online metrics are profit, revenue and audience. A lot of businesses are valued on 1-5 times revenue. A typical value based on profit is 15-25X. Growth rate is a big factor on the final multiple.
Go to Yahoo Finance, enter GOOG for the ticker symbol and then click on Key Statistics in the lower left. Google is currently valued at 5.5 X revenue and 21 X recent earnings (both high because of growth rate).