I'll start things off. When I've used recruiting firms, they get something between 15-30% of the first year salary of a successful hire. I have the conceit that you'd get better applicants via the HN ad than a recruiting firm, so I'll value a successful placement at $30k. On the other hand, a job listing on a jobs site could be as little as $100, but this low cost would be due to a lower quality of skill in the applicants and a lower chance of actually making a hire. I'll swag the chance of actually making a hire due to the post at 1/3, so I'd say the final auction price would be $10k.
