It's not cheap if you don't get any replies. I don't know anyone who clicks through on the paid placements (ads) - being the most popular site is the only way to go.
That being said, how does one become the 'most popular site'? Here's a hint: the FR forums have a reputation for being addictive. It's a combination of breakings news and commentary.
When I launched two commercial sites last year, objective #1 was to get to the top of Google's rankings. The chief tactic was to emulate FR, F**ckedCompany, SlashDot, et al for their particular niches.
Both the mission and techniques have been accomplished. The two sites avg between 100-150k page views (not hits, otherwise the number would be 2x) per month. Sales-through at one of the sites has been pretty good (downloadable practice guidelines).
If fact, I've so saturated the market for anything to do with accounting regulation, oversight, etc. that the other day I was searching for something only tangentially similar and the first few hits where my own sites.
In case you're curious, the two sites are Sarbanes-Oxley and PCAOB-Online.