No such thing as live-out au pairs. Live-in au pairs are normally around £70 for 25 hours (not usually sole care for under 2's). Plus accommodation, food and usually help with transport costs (bus pass/use of car) Nannies - depends entirely on area and demand. Inner London can be up to £12 per hour net though if you outside the south east you may find someone for as little as £7 net (you need to add on and pay the tax - go to listentotaxman.com for a calculator). Iy can get very expensive. Mothers helps are around £8 per hour but can be cheaper. They are good if you only want a few hours here and there.
NurseryWorld: Annual Nanny Pay Survey (Feb 2011) - I am not clear as to how this data is collected, so while it is a snapshot of nannies salaries I don't feel it will represent every nannies salary, as with many surveys it relies on people to complete the survey form.
Currently there can be a lot of nannies looking for work in some areas, so if you advertise a job at a salary level you feel you can afford to pay, then you see what applicants you get. No point in seeing nannies who want more money than you can afford - where would the extra money come from. Keep in mind when deciding salary that there are additional costs to consider on top of the salary - see Total cost of having a nanny (April 2011)
there are live-out au-pairs, I know the agency who specialises in them. I live in NW London. As far as I understand live-in au-pair is about 70-90 pounds per week for 25 hours (I pay 80) and live-out au-pair will be about 120 pounds per week.
Of course as mra points out the level of care you would get for that (no experience and no qualifications) would probably be a very limiting factor and you're not just looking at what people would generally perceive as an au pair for that money - any unqualified, inexperienced 18-20 year old would get the same.