The reason Cameron does these speeches is not for the money - he is already independently wealthy - he was in the Sunday Times Rich l List in 2009 at £10 million, before he became PM.
It's simply because the ego-massage, the endless fawning attention, the constant advice of wonderfulness, of influence, of wisdom, of intelligence, and over-arching need to be seen to influencing the world, is powerfully addictive, and politicians find it seriously hard to give up when the days of power are gone.
When that power was ultimate, as for Cameron and Blair, coupled with an ingrained sense of narcissism and perceived intellectual superiority - the need for an audience becomes as oxygen - necessary for life itself.
Add in the fact that outside the UK, you can enhance your achievements, value, and reputation out of all recognition to anything your previous 'subjects' would recognise, you can trot round the world trousering seven figures to get on your hind legs and talk garbage for an hour.
It doesn't matter that you were utterly unworthy of your position, that your electorate wouldn't recognise your current pre-speech brochure as being about you as they remember you in a month of Sundays, that your views and opinions on world politics are about as relevant and interesting as your views on the dark side of the moon ...
None of this matters at all.
It's the adulation, the rapt attention, the being listened to, admired, thought of as fabulous ... I reckon Call-Me-Dave and Tony Liar and Grey Major would do the speeches for nothing - it's not about the money any more, it's the inability to let go of that feeling of power, however non-existent it may really be.
God save us from ex-PM's - yesterday's men who have lost whatever shreds of dignity they may once have possessed, and bore foreigners, because the UK electorate wouldn't tolerate their fees, or indeed, their irrelevant, patronising perceptions of themselves as 'statesmen'.