How much does it cost to process a request to display a widget?

Question

Project management

Question to those who calculated the economy of the Internet.
Let's take a widget embedded in a website. For example, Facebook or Contact social plugins, or the Disqus discussion platform.
These are iframes that load JS and other code from the site. The code authenticates the user with cookies, makes a query to the database, finds out if there are, for example, new messages, and displays the widget according to the request. For example, there are 2 new messages.
How much does one display of this widget cost? How much does a database query cost? What is the economy, what is the cost.
For definiteness:
1. The result is needed within 0.5-1 second, no more. (Advertising messages are no more than 0.2 seconds, as far as I know.)
2. Widgets show on the computers of users around the world
3. We take a stream of 5000 impressions per second. This is for a medium company (like Disqus). The data processing service is supposed to be "as a service" (server rental), without building its own processing center "hectare by hectare" in size.