I received an insightful question from a reader today.  Here's what he asked:

"Lets say we're comparing Kayak and Expedia, to make it simple, lets say they are both looking at exactly the same market, however Kayak doesn't mediate the transactions themselves and they just get a referral fee, but Expedia actually charges the cards and is the marketplace. If Kayak gets a $10 referral fee per $200 ticket sold and Expedia sells the tickets for $200 but passes $190 to the airlines, also keeping $10, then Kayaks addressable market is 1/20 that of Expedia, even though both companies could have identical Gross Profits (Sales-COGS)"

Not all revenue is created equally.  As is especially the case with companies that sell a product or service on behalf of another party the revenue can be calculated two ways.  There's the gross revenue - which includes the money that is paid back through to the primary service provider, which is Expedia in the example above.  And then there is the net revenue which excludes revenues that are directly passed through to the underlying service provider, which is what Kayak books in the example above. 

In the end both companies effectively bring the same amount of money into the company:  $10 per ticket.  The net revenue is the number that really drives the business and therefore it is the number that should be used in your addressable market calculations.