Independent writers have different ways of charging for services. Most charge by the hour or by the project, some are paid by the day or week, some by the page, and, in long-term relationships, some are paid a monthly retainer. Some charge different fees depending on the service they're providing. You should know what you have to spend. Bear in mind that an hourly rate by itself is not a measure of total cost.
Some writers work faster than others.
You should also clarify the expenses you will cover. It is perfectly acceptable to ask a writer to estimate a total price for your job. It is also acceptable to ask for a written bid. However, unless your project is particularly large or important, it is unprofessional to ask for a detailed proposal unless you are willing to compensate the writer for the time spent doing it. Be prepared to discuss your manner of payment and a payment timetable.