I challenge the premise.
You (not your boss, or your client, or your employer, or your Mum) determine what your time is worth.
The first step is figuring out what your time is genuinely worth. Be honest with yourself here.
After that it is a case of putting that figure to your current employer.
They’ll either meet it, or they won’t.
It will largely depend on whether they need you more than you need them.
If they meet it then great, take the promotion and carry on doing business with each other.
If they won’t then your current employer does not value you as much as you believe you are worth. That tells you everything you need to know.
If your current employer won’t meet your price then you need to be prepared to take your bat and ball elsewhere. Providing you have set your market value realistically, then you will readily be able to find alternative employers who will pay that figure to utilise your services.
Don’t forget to factor in fringe benefits like flexible working hours, being able to work from home, and so on into the equation. Trading a few dollars for the ability to manage your own time is often a trade worth making.
Ultimately you want to choose the option that will make you happy, the rest is just detail.