It all depends on the type of assignment. It is not the same product, studio, artistic, etc. photography as documentary or social photography, of events, etc.
I think that in any case, you can use this model:
I make an estimate by the hour.
I establish a price per hour for smaller jobs, such as attending meetings, travelling, etc. This ensures that you don’t lose money with all that logistics and production work that is often not calculated.
I set another average price for editing and photo retouching jobs that is more mechanical. What in advertising and design is called “artefinalist”. Work for which you could hire someone instead of yourself. It’s the work you’ll probably spend the most hours on in a job, but not the most difficult.
Finally I set a higher hourly rate for creative work. Idea design, lighting design, creative proofing, and the photo shoot itself in case it’s in the studio. This is the work that really makes the difference and distinguishes one photographer from another.
The hourly rates I have are not easy to find, because they depend on your ability, confidence in your work, experience, where you live, desired standard of living, etc. But by looking around and asking your colleagues, you can start with something, and modify it as you see how it evolves.
Taking this into account, you estimate the number of hours you will devote to each thing, you add the expenses you will have (gas, material, equipment rental, study, etc.), you calculate some percentage of margin to higher, or some bonus or adjustment for the client if you consider it, and that’s it.
You can also offer your client a special price, or in batches, if you are assured of a certain amount of work. For example, in a festival with many activities, you may have to go several days to document different activities. I sometimes offer a fixed price for the documentation of each activity, in exchange for being assured of a minimum amount at the end of the festival. I ensure well paid work in conjunction, and give the client a good price.
Make sure your client understands all this, don’t be afraid to break down your budget and defend all the work you do. If the client then asks you or you decide to adjust your budget downwards yourself, start from the awareness of what you really decide is worth.
Don’t be afraid to estimate a budget and close it when you’re really done. You may not know how many things will turn out during the process. A good and reliable client will understand what an estimate is, and that the result may fluctuate slightly when the project is finished.
And above all, be honest with yourself, with your clients and don’t steal from people, but don’t work with anyone who puts obstacles in the way of a decent job.
I’m fine with that.