Pricing for different tasks that require the same amount of time from you tells the Customer (and your subconscious) that you're working at a 5 on task x, but working at a 9 on task y simply because it costs/earns more. That seems to be a disconnect. Your time is your most precious asset, and I would charge for it whatever you're doing. If you build a site, and they are happy with your dev fee, but feel like you should charge less for SEO, simply let them find another SEO guy. That's their choice, but YOU are worth $xx.xx, no matter what you're doing.

Also, in general, take whatever you're charging and add 10% to it. If you're still busy, add another 10%. Let the demand level determine how much work you do, and at what cost.