Zhang Heng's previous performances earned him quite a bit of money. Other than that, he also received a lot of gifts and copper coins from the excited spectators when he won his final battle at the Amphitheatrum Flavium. Although most of those gifts belonged to Mark Reuss, the money he earned was more than enough to pay the rent.
After Zhang Heng quit the gladiator school, Commodus almost instantly offered him a job. Simultaneously, he was still working for the Balance Blade, so he brought the slave around the town to look at a couple of houses. In the end, Zhang Heng chose the home most comfortable to live in. Its rent, however, was also the highest as compared to other houses.