It depends partly on what sort of SEO you have in mind. If you are thinking of on-page SEO, such as optimising for certain keywords, creating sutiable page titles, developing SEO-friendly navigation, etc., then those are things that should be baked into the site as you develop it, rather than added later. In that case, it's hard to justify charging extra for it. It's the sort of thing you ought to be doing anyway.
But if you are thinking of things like building high-quality backlinks, it might be better to offer that as an add-on. In that case, I would focus on getting the site finished to their compete satisfaction, and offer to do the SEO as a subsequent stage. By that time, they will be convinced of your skills, and will trust your judgement, thus making the SEO sell much easier.