Depends how well he plays. Obviously his old club are making a reasonable return on their investment but he has only got to have a bad season next year and his value and pulling power could diminish rapidly.
It is a long term investment. Kaka could have 6 or 7 years of top flight football in him. Staying in the premier league is worth around 60 million a year. You get one of the best players in the world then other good players will want to join. If they can rival Man U, they would be one of the richest football clubs in the world. Well Man U were till the flogged it and now the banks nearly own it. Those rich middle eastern gents who have made many billions on black gold have to spend their dosh on something.