Well, recently we were revising pricing of our digital marketing service packages and while bench-marking, we realized how widely spread the industry quotes for SEO services.
I suppose the issue is similar to How Much For a Website.
We should again define the scope of the SEO requirement, from keyword research consultation, to the initial audit of the website, to making changes to the website structure, to how many keywords to rank, to the contents provided for uploading to website and finally the maintenance of SEO ranking over time.
SEO is a long term effort. If the business has someone who is more technically inclined and also has someone to produce contents on an on-going basis, one good way is to part outsource and part in-source.
Consider working with your SEO agency to set up the initial SEO infrastructure and then have them train your internal resources to maintain your rankings on an on-going basis.
Good agencies quote $1K to $2K per month for maintenance of SEO, so it is perhaps quite worth the effort for the training of internal resources.
You get an idea of what is involved in SEO from How To Do SEO.