Different doulas charge different amounts and there is currently no regulation or recommended amount. On average its likely your birth doula may charge you between £300 - £500 - or your postnatal doula between £10 - £20 - if she's at the beginning of her journey she may charge expenses only. There are doulas who always charge expenses only. Most doulas will offer payment plans and sliding scales.
Generally most people caring for the birthing family want them to feel supported and cared for by someone that they know. Someone who has faith in them and the process and can follow them through the entire journey - through pregnancy, birth and the early days. Mostly our healthcare system isn't set up to provide this kind of continuity so in the main our maternity care teams are in favour of families being able to access this support via the doula.