Yes, of course. Someone could be earning £50k a year but have three children at university, and would therefore have less disposable income than someone with no kids on £25k a year. But the general correlation is that the more money you have, the more disposable income you have.
In my view progressive taxation doesn't start with the people who earn £50k a year though; it starts with the people who earn millions. I can see why people need £50k a year to live a nice life; I'm a bit sickened by people who earn millions and pay very little tax.
Yes, the need to earn a wage is a great motivator and any system should be designed in such a way that people are motivated. The UK benefits system is designed in this way, to a great extent - whatever certain newspapers would have us believe, it is very difficult to live off benefits for a long period of time unless you have a long-term disability. And the mount of money lost on "fake" benefit payments is nowhere near the amount lost in tax avoidance.
Anyway, this is now getting pretty off topic so I'm going to sign off. Night, all...