As a web designer who wants to promote or "offer" seo, the way you design sites and how they function should fundamentally be optimised for such.
The way the site is set out, document and folder names, pages titles, descriptions and the use of h1 and h2 tags and of course appropriate keyword selection will all help build a solid foundation to the sites overall PageRank performance. However as a web designer, there is only so much time you can allocate to research marketing and keyword selection and then of course copywriting! And of course dont forget a possible SEM strategy whilst you may be waiting for your seo efforts to take affect.
My advice to you would be to offer you knowledge in more of a consultancy type role, where your clients still provide content, but you simply review and modify it with your own research and knowledge in that field. Having said that, there would be nothing wrong in you picking up a new client who recently had their website completed and optimising it for seo. Then you would be performing the tasks of an "seo expert" and as such you can expect to be paid for your time spent on such tedious tasks.
Don't forget, for a client having their website designed by you, they may perhaps already feel somewhat overwhelmed with the process and mightn't understand the entire benefits of seo! therefore, it would be a shame to lose out on the job as it appears that you seem somewhat overpriced. Therefore, client selection is also a factor to consider.
In closing - whilst site engine optimisation is an ever ongoing process, it is at the design stage where it begins (you should highlight these points of difference to your clients) furthermore for knowledge, never stop reading and as a start the Google Webmaster Guidelines is a great place to begin. Then purchace any book on the subject as it will always cover something else that another book may have missed.