There are a few things to consider when determining how much to charge for your freelance writing services. One is whether you will charge by the hour or by the word. Each has its own advantages and disadvantages.
Charging by the hour may be more predictable, but it can also lead to less work if you are not efficient in your writing. It can also be a good option if you are just starting out as a freelance writer or if you are not sure how long a project will take to complete. Charging an hourly rate will give you a little more flexibility in terms of how much you ultimately charge for a project.
Charging by the word may be more variable, but it can also lead to more work if you are able to write quickly and efficiently. This is especially true if you have a lot of experience as a freelance writer and you are confident in your ability to complete projects quickly. Charging by the word can also be good if you are writing for something like a video game blog or website, where word counts tend to be relatively short.
Of course, sometimes you simply don't have a choice. You may need to accept whatever pay rate the publication offers, or you may need to set a flat rate for the entire project. These offer less flexibility but can work out in your favor.
The decision of how to charge should come down to what makes the most sense for you and your client. If you are able to negotiate a fair rate that meets both of your needs, then that is the best option.