We can safely assume that Polo wasn't motivated by greed or monetary compensation when he undertook his voyage but rather by spirit of adventure and curiosity.
There are several reasons why money was not an issue when he decided to propagate his knowledge, but first and foremost it wasn't an issue because his family was already well-off else he wouldn't have been able to travel through Asia (as his uncle and father did too).
Furthermore I doubt that the book would have brought him any advantage, as he would not have been able to capitalize much on his knowledge. Apart from the fact, that you would have bind a huge amount of capital over long periods of time (land travel on foot from china can take several years), establishing direct trade cutting off all intermediary trade also brings huge risks. Venetians didn't need that, because the products they traded with were mostly luxury products like silk, which they could get enough of a profit in by importing them (already with high prices but without the risk and the voyage) from Arab and Crimean intermediaries. Cutting them off would also have deteriorated their relations...which brings me to the next point:
Direct trade between china and Venice was a political and logistical impossibility at the time. Travel through land required permission and support of a vast number of lands and rulers, which often weren't stable. They would have to establish permanent contact with every one, send emissaries and probably bribe more than it was worth, and it still would be too long a way to effectively guard, control and secure.
Travel through land also requires large, bulky, exposed and very expensive convoys and caravans, for what was traditionally done by short range intermediary merchants without a supply problem.
The other option would be by sea, but the Venetian Fleet wasn't equipped with the knowledge, supply ports and logistics and the ships to sail around Africa - mind you, the Suez canal wouldn't be built for another 5 centuries.
The countries you cut off the trade were countries which based their wealth on spice and silk trade, so they wouldn't be happy either - and you need to pass through their land.
Another important point is, that Marco Polo was in prison while he dictated the story. Being a Venetian in a Genoese prison generally wasn't a good sign for your personal commercial enterprises. He eventually got out, but I don't think he ever regretted it.
Because last but not least...probably what counted more for Marco was fame, or a sense of duty to pass on his knowledge or some other intangible motivation. Money he had enough.