The only way you will be able to earn good money is if you are clear about the scope of the project. This apples from the beginning, and you stick to your guns.
You need to know important details, like how long the written piece has to be. You also need to know when the client wishes it ready, and how much you will get paid for your work.
In the eventuality that the project is huge, it is always recommended to get paid a retainer to secure your services. Once agreed, you can then set milestones with the delivery of certain parts of the project and get paid. If you don’t do this, then you might have cash flow problems.
You should be on the lookout for your client starting to make other requests and to add other elements to the already agreed job. It makes sense to agree to these requests if they make sense and once you’ve gone over and updated your contract.
Don’t let your client bully you into anything which you feel unfair about your work, time, and skills.
Your client should appreciate your professionalism by respecting your skills and what you are worth.