It's all expensive.
Even a small single room in a flat/house share at commercial rates is upwards of £600/month, plus bills plus council tax (unless you are a student, or otherwise exempt).
For anything decent, plan on £1000/month
Many people will group together and rent a house, when you might be able to get something a little better for th emoney (though rarely cheaper).
IN general, the bottom end of th emarket is where you are competing for room space with students, so the issue of Council Tax (students don't pay, but employed people do, and you may end up paying for the whole house, if your fellow sharers are students).
Also, the whole housing scene in London is full of scam merchants who will take a 'deposit' but then you never hear from them again.
To answer the question, anywhere with high-density housing that is also a long way from a tube/rail station and with a journey more than an hour tends to be cheaper.
As said above, the ends of the tube lines tend to be cheaper. South tends to be cheaper than north. East London tends to be cheaper than West.
Croydon has good communication links, and tends to be relatively cheap - look for areas like Addington, THornton Heath, Mitcham.
Good luck. It won't be easy....