I go out one day, full of beans, weather is good, bash out £350, happy with that.
Next day I might go out, it's grey, drizzly, I really can't be bothered and hit £200 and pack it in.
Don't worry about what anyone else is earning, earn what you need to maintain your lifestyle.
I have slowly learned to appreciate this job, not so much the cleaning of windows itself which is mind numbingly boring, but the flexibility and ability to earn a decent wage whilst being an absolute lazy sod when I want and working hard when it's in me.