I think you should charge an hourly rate, but maybe a little lower, half of what you normally charge since your client is going to provide the training process. However, you are using your time to learn it so that you can provide a service for him.
Learning
SEO is quite a challenge but once you have the knowledge you will be able to add that to your resume and charge higher than you are for your regular services.
Good Luck
Robin
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Virtually Robin