You are tying here the income of the papal central authority to indulgences. But it was not just that. It was a lot more.
In the early XV century, the catholic church was an efficient money making machine. Each of the cardinal possitions, each bishop's appointment formed an incredible chess game where moving each piece, even the minor ones, produced a rain of gold in Rome.
Not only the pope got income from indulgences, but for cardinal, archbissop, bishop and even minor clerical appointments. And it was not cheap. Moreover, the church controlled extensive holdings in every catholic countries, and it was a true State-within-the-state. And that is one of the reasons the German princes turned protestant.
The tithings and many of the taxes from this lands and business went to Rome, since in many, if not all, catholic countries, the religious holdings could not be taxed by the local king.