Getting rejuiced in US politics, even if he didn’t get paid directly, made sense.
The key question I think, is “How did Manafort expect to be paid?”
Manafort reached out to Barrack in January/February 2016. It was far from assured at that point that Trump would win. He came on board as Trump was worried the GOP establishment would deny credentials to his delegates. In fact, one of the key points of emphasis about hiring Manafort was his ability to count delegates back in 1976. In January/February of 2016, or even in March when he came aboard, there was plenty of establishment uneasiness with Trump. Given the desperate need of Manafort for money, it seems incredible that he would accept a non-salary position for some future payoff that depended on a more than a few lucky breaks. The assumption was that Hillary would win – Trump was too clownish to possibly succeed. How would working as the campaign manager of a failed presidential candidate help him? Again, Trump was not the choice of the establishment. Manafort brought no skills that would endear him to a sinecure at Heritage, AFP, or some other winger welfare circuit in the event Trump lost.
Given what Manafort did in Ukraine and elsewhere, there were more than a few despots and autocrats willing to pay him for some good press in the US. So why didn’t he take those opportunities? The Trump decision only looks good in retrospect because Trump won. That was far from assured in March 2016, so what was it that steered Manafort away from his 30 years’ experience with foreign clients to take on a buffoonish GOP candidate?