Doing as suggested and just raising the price until your conversion rate goes down, is definitely one way to go shortterm. It is a simple approach, and you determine, what is the optimal price point.

However, it feels like more value is hidden here. A few ideas:
1. Think about changing it into subscriptions, so 4 postings a month cost f.ex. 50$ (or more depending on your newfound price point). Your model is very transactional, and a subscription model would give your more stability earnings; your customers would pay for capacity, not just for each individual post.
2. Understand what your advertisers get from posing through you. Traffic? Leads? Charge accordingly; leads are worth much more.
3. Understand your followers: Do segmentation, and offer analysis of the segments, that follow individual postings for an additional fee. You have monopoly on those data, and I am sure, that the businesses will pay you to understand, who your followers are and what they like.
4. Develop you model, so they pay for traffic: 10$ if you reach 5000 on day 1, and and additional 2$ for each extra 1000. They pay for more gain, and lower gain will not cost them.

Good luck with it! And reach out and let's talk, if you want to dive further into any of these ideas.

Best regards
Kenneth Wolstrup