First of all, the client and you need to decide whether payment should be fixed price or hourly rate.
Clients automatically tend to prefer fixed price - but you should only accept this if the requirements are also fixed or you have performed similar tasks so often that you are aware of the actual work load.
Freelancers tend to prefer an hourly rate to avoid being 'guilted' into doing free work at the later stages of a fixed price project.
In addition, an hourly rate means the client will always evaluate whether they actually need a specific feature, whereas they under a fixed price often will try to extract as much work from you as possible until you draw the line.
What your actual hourly rate should be, depends on your circumstances - but you obviously need to get paid more than a similar full-time employee.
If I have to estimate the price for a fixed price project, I take my hourly rate and multiply it by the time I believe it takes - and then multiply it by a 'risk factor'. You should tell the client that a fixed price comes at this premium - as that can often nudge them towards accepting a pay-as-you-go hourly rate (if that is what you prefer).