By month and allocation of tasks. If you charge by keyword ranking, you won't have a guarantee that you will make money, as it is easy for SEO to fluctuate based on the conditions of the time and updates that happen as well as the dynamics of the website.

If you charge by month, identify which tasks that you have completed and offer to add in audits to showcase how the keyword rankings are changing. SEO is more than content, so add in different types of services within SEO, such as internal / external linking and how much, optimisation of content, technical services for the website. Define how much you want to charge with each of these.

I've worked with SEO clients for over 12 years and can help you too build your business as well!

Hi
You ask a good question. This question is similar to when startups ask me which would be the best business model or pricing mechanism.
My answer, which might simple simple (but read on) is: both.
People like to have the option to choose. It lets them feel more in control, especially when having to pay for services. I have personally used seo services in the past, and never agreed to pay for the per month option as too many companies were abusing this option and it comes with uncertainty for the client.
So, what you should do is create two payment options: one based on results (keyword ranking) and one based on the per month method.
A few important things to note when doing the above:
1. Do not offer the results based option if you don't think that you can reach results due to limitations/conditions on your client's websites.
2. Make sure the results based offer reflects all your costs, the risks and the desired profit that you want.
3. Specifically for SEO services, make it clear (upfront) to your client that they will need to make certain changes to their website for your service to work.
There are a few other issues that you need to cover. I will be happy to discuss these by phone.
p.s.: if you added the link in your question for SEO purposes, then: on the one hand - good job. On the other hand, for people who understand SEO, they might think that you're using black hat methods and then decide to refrain from using your services.
Good luck

Charging by keyword is an old tactic which can backfire since there is no guarantee how much time it will take to bring up a link on the first page. Getting new resources and pages on Google's first page has become highly complex.
I run a marketing agency and we have a lot of SEO projects running. We charge all our clients on the basis of work done on a monthly basis. If you will do good SEO work for a couple of months, the results will start trickling in in the form traffic and engagement. Use these improvements to prove your work.
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Good luck with your work!

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