This is almost a contradiction in terms, except when there are no events on!
When there's a big event, high demand pushes prices up. Planning and booking ahead is likely to result in the best balance of choice and price. And, of course, how much you can afford is always a consideration.
If you're travelling on a budget, consider Couch Surfing – it's affordable and sociable. There are no campsites for miles: the closest is at Crystal Palace. Roadside parking may be possible a few miles out of London for motor homes – one option is the overnight ticket at Heston or Scratchwood Services.
If you are willing to travel, consider staying at the other end of town. When Excel or the City are quiet, their hotels may well be affordable – and it's a peaceful place to stay. A daily travelcard will cover this or other longer trips at a price of £6.70 per day (2008, valid after 9.30am). Click here to see London Excel Hotels.