I think that your question is a little too broad but let's see if we can narrow it down. Based on the unknown, CPA = Cost Per Lead divided by Conversion Rate. However, these metrics are mostly applicable in the SEM world when advertising using PPC models. In my opinion, your CPA is undefined as it stands. The more important question you need to ask yourselves is the monthly paid media budget you'll need to invest. There's a different formula for that. Typically when considering a PPC advertising model, budget plays a factorial role until you have available metrics that can help you determine your Average CPA. Hopefully this helps. Let me know if I can be of further assistance.