Ask HN: Monetizing vertical search by charging for premium content?

I am trying to find a practical way to monetize a free Google-indexed vertical search engine that already has traffic.

Has anyone tried to monetize a vertical search engine by charging users for premium content?

For instance:

Let's say you have a free vertical search engine for real-estate, but you charge a yearly subscription fee for the ability to download scanned TIFF images of quit-claim deeds from the county courthouse.

Has anyone tried something like this? How well did it work?

There are many pre-Internet "vertical search" engines which seem to charge for content: Westlaw, Lexis-Nexis, Bloomberg, IHS, etc...

Are there any examples of more recent vertical search engines that monetize via subscription?