A Doula salary can vary a lot as the work a Doula does is essentially freelance. A Doula salary is, therefore, entirely dependent on clientele and rate that the individual Doula charges.
A rough idea of Doula salary can be gathered by working out average Doula cost per hour and the rough amount of work a Doula gets.
In the states a Doula salary can be roughly $30,000 USD per annum.
This is a conservative estimate and a Doula in high-demand with steady, regular work could earn anywhere in the region of $45,000 USD plus.
In the UK Doulas earn around 10-15 GBP per hour when working and charge a flat fee for attending a birth. This fee is usually in the region of around £350 GBP plus to be present during labor.
Again you could attempt to extrapolate this out and find a rough Doula Salary UK but true earning will be entirely dependent on how many clients you have, what you choose to charge and how well you market your services.