Before we get to the ending, let's jump back to the beginning for just a second.
Tropic of Cancer starts in the Villa Borghese, with Henry's friend Boris and some lice and some hunger; and then it moves into a pervy love song to a woman named Tania. It's pretty grim in an everyday sort of way.
By the end of the book, though, our guy has gotten pretty poetic. Plot-wise, Henry has just managed to smuggle his friend Fillmore out of the country and back to America. That effort has led Henry to reflect on the possibility of returning home himself. But, of course, we know he won't do that, given that he's just spent the whole novel being in love with Paris—for good or ill.
The novel closes with Henry down at the banks of the River Seine, counting the money he has basically stolen from his friend:
Human beings make a strange fauna and flora. From a distance they appear negligible; close up they are apt to appear ugly and malicious. More than anything they need to be surrounded with sufficient space—space even more than time. (15.112)
That's a little different from the penis elegy that started the book, right? Though he's getting pretty metaphorical here, he may be suggesting that Paris is a city that gives him the sort of mental, artistic, emotional space he needs.