As I suggested, estimate the effort for initial setup in terms of man month. Similarly, estimate the effort of ongoing optimization. Accordingly, propose an amount.
One way to validate your thinking is to approach an SEO company with similar requirements and ask them for a proposal with quote :). You may even consider associating with such SEO company if the project requires multiple skill sets, which you don't have. However, do ensure that you don't end up wasting significant time of theirs.