Well you needs severs that can handle the 10,000+ QPS that they are going to send you and you need to write code decide what you are and are not going to bid on, and their is no trivial work to set this up so they are going to think that you might spend lots of money in the future.

You could play with https://contributor.google.com this works though Google's exchange and the gist is they bid on your behalf for ads that will get shown to you, you can then put arbitrary HTML in those ads. I saw one guy that was learning Japanese so he used it to display vocabulary words in place of ads. The major downside is it only works on ads that go to Google's exchange which is kinda random on most sites because of stuff like the article above. It ends up being a couple dollars a month to replace random ads on websites.