One of the problems with being a writer (unless you work in a cubicle as a writer) is that you never really know how much you are going to make for each hour that you spend at the keyboard. And if you are supporting yourself as a writer, it is something that you desperately need to know.
(This also applies to all professionals who are self-employed, so bear with me if you are not a writer.)
Robert Heinlein, one of my writing heroes, used to have a policy that he never wrote anything that he wasn't going to get paid for. Now, he did do a certain amount of writing that he never unloaded (aka sold); but for the most part, he did a pretty good job of keeping to that policy. So much so that he managed to pay off several mortgages as a writer.
Now, before you view that as a sign that you too can make it as a writer, just remember that when you start looking at the amounts Heinlein got paid, he got paid in the 1950s what writers are still being paid...and the cost of everything is higher today than it was back then. Writers are still being paid by the standard rates set forth in the 1950s--I suspect that the reason why is that so many writers are willing to work for free just to see their bylaw in print. Furthermore, Robert Heinlein was a really good writer.
Estimating how much you are going to make is easier if you are fulfilling orders that you know that you are going to get paid for (especially if you know the rate of payment). Freelance writers like having clients who ask them to do regular batches of work for them. Unfortunately, one of the things that could happen is that you find yourself working long hours for too little money.
I recently got cut loose from a project that was chewing up way too much time for way too little money. I could have made more per hour if I went back to flipping burgers. The appointed tasks essentially got worse and worse as the project dragged on. Needless to say, it was one of the jobs that I am grateful that I got fired from. Besides I know that I can make more money doing other types of writing. (Why do I know that I can more doing another project instead? Because I know the spread on how much I am making on various types of projects).
What a writer needs to do--on all their projects--is to keep a time sheet, and track how many hours projects take them to complete. The amount of money that one makes from a project then needs to be divided by the number of hours one worked on a project.
(This applies to all professions--keep a time sheet--figure out what projects are giving you the most amount of money per hour--it will do your business a world of good if you do so.)
Now, for me, a lot of my writing is of the page view (you get paid based on the number of readers that your article gets) and royalty based (you get paid based on the number of copies sold). So not only do I have to keep track of my hours, I also have to keep track of all the monies collected over a long period of time. In other words, a lot of my stuff is paid for (not upfront) by the level of readership I end up with.
(This long-term collection of payment information applies to anyone who is selling the same piece of work over and over again.)
This is why one of my current jokes is that as a writer, I make somewhere between zero and thirty-five dollars an hour...some projects I have received so little money I have yet to earn my break-even point yet (my "burger flipping" wage), and on other projects I have earned hundreds and hundreds of dollars already.
I am getting better at just doing the type of work that have earned me the higher amounts based on previous outcomes. (Occasionally, I do miss because of changing markets and the continued need to expand my writing into unknown [never done before] areas.)
As I said, what I am having to do as a writer--tracking hours spent working, and overall income received--applies to other professions that do not have a clear cut hourly rate. It is worthwhile to know your estimated hourly rate on various types of work to be able to budget your earning time more efficiently.