That's just the point - he wasn't paid market value. ITV have gone on record as saying they don't pay their top stars anywhere near what the BBC does and, given the current ad market, they simply can't afford to compete. The people who are earning massive money on ITV are doing so by owning the formats they are on and franchising them and/or using phone in voting neither of which would be available to Ross as there's virtually nothing original about his shows he could claim ownership of. The fact his C4 show has fallen though must partly be due to his wage demands, because talk shows are otherwise incredibly cheap to make. Nobody else is willing to pay Jonathan Ross near what the BBC were prepared to pay him.
Of course, he's not alone. The BBC pays an awful lot of it's staff (especially the front of house talent) very well indeed. It just doesn't seem willing to spend it's own money on the gruntwork of television, thus (for example) the hiving off of what is now Red Bee Media or the more or less abandonment of the Dirac codec research.
