The underlying reason for those services having both high costs and high revenues is that they have extremely high volume of users. I'm sure something like Duck Duck Go has much lower operational costs, even though it's providing not substantially different service than Google's search engine.

My point is that the cost and revenues are not a function of each other as much as they're both a function of usage. Once you get a high volume of users, it's pretty natural to convert at least a small part of that to advertising revenue, even if doesn't fit well to your platform (cf. Twitter). It takes a principled decision not to dip into that revenue source -- I remember once reading how Jason Calacanis was pissed at Jimmy Wales for not wanting to include ads on Wikipedia.