For example, if my work creates 30% more monthly net profit.
Ideally, you should always charge based on the value you create. Try at least to think about or talk to your clients about how to quantify the value you create.
But your client wants to be treated fairly, which is also very important - but is very subjective. No matter what value you create, sharing the value creation is a very rational process. And customers are not always rational like that; they may hold an idea of how well you should be compensated.
You should also be aware of the market price for your service - f.ex. if you are working in consultancy.
So think about these different components: Measurable or quantifiable value created, clients assumptions of fair price and market price level for your service. The more alone you are with the service you provide, and the more you can persuade your client to consider you an investment in value creation - the more you will be able to capture the value.
But be sure to agree on it up front, so your client does not move away from the deal after you have done your end of the job.
You have chosen the harder road to the goal. The easier ways are just to price based on cost plus margin, or adopting your competitors price structure. But the value creation path is the right way to go. So don't give up, but also accept, that this is a learning process; you won't be able to hit it the first time, you do it. Luckily, the game is repeated.
Should you need some dialogue through this process, feel free to set up a call.
Best of luck to you.
Best regards
Kenneth