If you're negotiating a price for a work that's to be "written under contract" the buyer should be willing to give you at least a rough indication of what the preferred method of estimation is, including a range of hourly rates or word count rates, if either is how they're willing to pay.
Since only an idiot would offer to pay by the hour without specifying a total NTE (not to exceed) amount they're willing to pay and defining a "scope of work" for a tentative/potential product, it's usually fairly easy to adjust actual hours worked against the amount available to come up with what the buyer will accept in terms of hours billed, if you're forced to guess a bit during the course of the work. If you're dealing with someone who has any reasonable amount of experience with using OP people, they'll know, and should tell you, what their common rates are.
Note that it's important to understand things like: if manuscripts will need to be exchanged, shipping a draft proof paper copy of a small book to meet a deadline can be $100 within the US, so knowing who's paying can be of interest.
Among contract workers (job shoppers) the only real way to know if you're doing as well as others is called "NETWORKING," and it's essential that you make friends and share, with others doing similar kinds of work, information on managing, marketing, and getting paid for your work. That can be incredibly difficult to do when first getting started - or restarted; but it's really part of the job.
Most job shop and/or freelance work is not a situation where you can demand a particular hourly rate - or depend on averaging a particular amount, although you need a "target" to work to. It's often a matter of you telling them what rate you usually charge, with them ignoring it and telling you what they'll pay.
("On my last job I received $xxx per Yyy, and that rate would be acceptable to me for this one" might be one way of soliciting a counter-offer, from which there may or may not be room to negotiate.)
(("Oh, I didn't understand that you were looking for a half-assed job. I can do that too." is probably not a good response, if you receive a lower offer than expected, even if it's true.))