Ask HN: How much traffic does a job board site need for paid listings?

I'm working on an educational content-centric site as a side-project. I'm considering partly monetizing it with job listings.

Assume there are different areas on the site for everything from computer science to math to art to physics etc.

How much traffic to the relevant area would I need (e.g., computer science) in order for a company (e.g., Google) to consider paying for a job posting?

I know this is still very vague, but any ballpark numbers?