My "offline" freelancing work quotations are structured like this: Firstly, I have a target of how much I want to make in a month, secondly an idea of how many hours I'm willing to work in a month to reach that target, and finally an indication of the delivery requirements of the project, if any (some quotations ask me to specify by when they could reasonably expect delivery, while others give an idea of when they require delivery). Then, I balance hours vs money vs delivery. So a project that has a delivery of a month (or multiple thereof) and means spending mostly all the monthly hours on that project will attract a rate that means I get out what I intend to for a month. Something that has a shorter delivery requirement with a higher number of hours to be spent (those projects where something needs to be done urgently, for example) the rate is a lot higher since more hours are spent on the project. I also work in a setup cost for each project, which is not billed directly but rather included in a higher hourly rate. I'll typically make more from 4x 1-week projects than 1x 4-week project, but the latter at least getting me to my target. While the former means a nicer "paycheck" at the end of the month, it requires more effort to get right, and it means requiring 4 different projects.
Obviously in practice there's other issues to consider, but such things are picked up over time. For example, projects don't occur one at a time sequentially. Which means sometimes working on 2 or more projects concurrently (your choice completely though whether you accept the extra work and whether you've planned for it). This means when sending the first (and subsequent) quote out, that the 'weights on the scales' have been adjusted accordingly. Then there's the consideration that there might not be work for some period, in which case either the end monetary target needs to be adjusted down for the period without work, or otherwise have an inflated monetary target to begin with, the excess of which caters for those periods (you'll need to determine what is a fair increase and how long it will take to build up a sufficient amount before you can afford to be without work - that length of time is a good indicator of savings needed before starting freelancing). Something else to consider is that a sufficient amount of time will need to be spent on admin work: generating quotes, responding to emails, invoicing, following up with payments, meeting (potential) clients , etc. You'll need to decide how to bill for this time as well.
Aaand I'm almost done with this novel... since you're wanting to do freelancing work online, you'll need to consider most of the above, but importantly, that a) your success rate of landing work is likely going to be a lot lower than what you think, b) the amount of "admin" required increases (responding to and browsing alerts, etc). All factor in on the total you'll make in a month.