I provide conversion optimisation services on a price per day on a rolling monthly basis. I did it this way, because my background is in software development consultancy and everything was estimated and billed out on a daily basis. I also provide one off services which is normally priced based on how long it would take to complete. I prefer to work with customers on a rolling monthly basis because I can have an impact on many aspects of their digital marketing and business processes. It means I'm also not tied to only creating split tests but have the freedom to advise and have a positive impact on multiple areas of a business.
I guess it depends on your business model, but I prefer to go with an approach similar to what Kevin has done. My business manages the entire process from copywriting and design to coding, deployment and conversion optimization, both for desktop and mobile landing pages.
If you want a scalable business I'd recommend moving away from charging by the hour. There are only 24 hours a day, so that automatically limits your potential for revenue. You will be better off selling a 'product' that gives you infinite scalability.
I charge by month with an additional small startup-fee for the initial setup. That it's easy for the customer to know how much they pay for the monthly optimization while I make sure to get paid for the initial work.
Hope this helps. Feel free to call if you need more tips and advice.
I used to sell similar services and now advise a company that works in the optimization space. I agree that monthly fees are ultimately the way to go and that the connection to your time spent should be loose at best. The main price driver should be to the ROI you deliver. One thing to consider, an option with a more modest fee in return for a "bonus" related to the creation of incremental revenue and profit. Most companies won't bite but it shows you believe in your services.
Having the monthly fee is great - but something I found to be unrealistic in the beginning. For me, it started out with hourly consultation pricing. Then once you have some results you can calculate, you can start charging daily or monthly fees. Which ultimately is what you want to disconnect your time spent from what you are able to earn.
An alternative would be to make a percentage deal. But that's something I would stay away from. Especially because very often clients don't implement the advice/optimizations I am giving them. Which in turn then leads to them not having the results I was expecting.
And as always: people value what they pay for. If you consult for free, they will not do anything with that knowledge. If you get paid, they, in turn, do execute.