This should all be taken with a rather large pinch of salt. I am not a business advisor / your business advisor…
It’s not really about your value, but the value of the product or service you’ll provide, and of course this is wildly variable!
Without knowing anything about you, my very rough algorithm for determining this would be something like:
Work out the scope of the task as best you can.
Turn that into an amount of hours based on your best guess.
Double that, because you will be wrong!
Now figure out what minimum wage for those hours would be in your area (or minimum acceptable wage, to you)
Now double that, because as an unsalaried worker you will have no benefits, leave entitlement, insurance etc.
So, you think it’ll take 1 full week, i.e. 40 hours?
40 x 2 x 15 x 2 = $2400
Then when the client asks for your estimate, you say 2400, without trembling or hesitating.
If that’s too much for them, ask them what budget they have for the work.
They say $300.
So you write a much smaller proposal for them outlining an MVP you think you can produce in 1 day, or whatever amount of time they are willing to pay for.
Or, you take the 300, but that way madness lies…
This is all based on the assumption you can actually do the work