It's a good question. I'd flip it around a bit and say that if you're a search engine that:
- Hosts commercial advertising content (not the entire web; only product and service offerings)
- Charges for hosting
- Has a highly qualified audience that genuinely wants to receive relevant advertising (based on requests being user-initiated, not software-initiated)
- Claims credit for revenue generated as a result of providing relevant advertising
... then you're likely to be able to cover the costs of developing and maintaining the system.
- This is a bit like eBay or Amazon, arguably; except that the user profile and context is optional for the user to provide
- Addictive products could still be a loophole that bad advertisers could exploit to monetize users without providing them overall benefit; fortunately local regulations tend to cover that
- Hosts commercial advertising content (not the entire web; only product and service offerings)
- Charges for hosting
- Has a highly qualified audience that genuinely wants to receive relevant advertising (based on requests being user-initiated, not software-initiated)
- Claims credit for revenue generated as a result of providing relevant advertising
... then you're likely to be able to cover the costs of developing and maintaining the system.