How to price a job board?
What is a right price at the beginning?
Try price it according to your competitors. You can undercut them (price lower) in the beginning to attract companies. I started with $1 per post for example. Now it's $299-$399
Really, $1? How does this not signal it's not worth much?
The right price depends on your goals. With a two-sided marketplace like a job board you probably want to kickstart the supply side (e.g. job posts) in which case it makes sense to charge nothing at all. Like @levelsio did you can start with a nominal amount to help keep out the spammers, but I personally would just charge nothing and manually approve new listings until that becomes untenable.
If you already have an audience then you don't need to solve the marketplace problem and you could start charging right away. However, even in this case you might want to start at $0 or very low just to test the waters and get some metrics on how well the job listings are performing. As that's information you'll need when you start charging real money.
Okay, so let's say you're ready to start charging real money. How much do you charge? Well this really depends on how much the customer is willing to pay. Which in turn depends on the value you provide (reach and relevance play key roles here), the competitive landscape, and a whole lot of other factors. One approach is to look at the competition, significantly undercut them (again, see @levelsio's advice), and then slowly keep increasing the price until you feel like you've hit the sweet spot. It's generally easier to increase the price over time then to decrease it (as that will make for unhappy previous customers) so that's why starting on the low side might be better. However, this also depends on market size because if your business will be primarily repeat customers then those might be less willing to pay a suddenly higher price. So again, it depends on the circumstances.
Either way, once you feel like you've found the right price: keep iterating over time. The market will change, the value of your job listings will too, so you should frequently reassess your pricing. (note to self)