Birth doulas charge anything from expenses only (some trainees do that and it is encouraged) up to - IN LONDON - £600 (I heard one charged £800 but not sure who that is). Many cahrge around £400 or £500. If money is an issue I would highly recommend a trainee (they are already trained but are going through an assessment process).
A postnatal doula charges between £10 and £15/h in London and I know a couple who charge more.
A birth doula meets the parents at least twice antenatally and keeps in touch by phome and e-mail. She attends the birt from whenever the mother needs her, this may be early on - hence some doulas can attend 72 hours birth!
The doula would stay with the mum until she was comfortably settled in the postnatal ward (if having baby in hospital)
She will see the parents once or twice after the birth -to debrief. Some will write a birth diary too... each doula is self-employed so she can decide what her birth package comprises of. The more expensive doulas are not necessarily the best or most experienced!
A postnatal doula works for the mother after the birth for a few hours a week (it may be every day but it doesn't have to be, usually for 3 or 4 hours). She will do all sorts of things, what she is not is a cleaner but she will usually happily help with laundry, dishwasher, meals, baths, school runs, what ever HELPS the mother in the first few days or weeks after the birth.
Feel free to contact me if you need more info
You can find doulas without paying an agency fee on Doula UK . All their doulas have signe a code of practice and are trained and monitored.