At $500 an ad a small company needs to justify that purchase - especially when finding someone may take six months and 8 ads. Of course StackOverflow Jobs won't be the choice for that sort of talent search at $500 per ad.
They should have made job ads $50 each and advertised the price up front.
It is worth noting that in the current economic conditions job ads are dramatically less effective because there is a shortage of developers and because recruiters are now contacting candidates directly. That said, job ads are not dead - job ads need to adjust to the market conditions. Job ads used to be worth $500, now they are worth $50 - that's how the recruiting market has changed.
At the moment it is extremely hard to find developers - this results in lots and lots of job ads.
StackOverflow management are clearly inexperienced/incompetent in the recruiting field if they can't capitalise on this in the current environment.
What a waste.