The success of a job board is based entirely on it's ability to attract candidates. So you've designed a decent site, priced yourself competitively now how are you going to attract a huge amount of candidates?

The other drawback of cheaper job boards is that more senior candidates tend to avoid them due to the assumptive association that they are essentially cattle marts for employers to find low quality talent.

don't sell candidates to recruiters

Most job boards, particularly smaller job boards, rely on recruitment agencies as their primary source of income. You would be missing out on a huge market here.