Okay, I was thinking Stannis went broke when he retreated from the Battle of Blackwater Bay. Apparently, the Iron Bank loaned Stannis money after that to hire the pirate a second time.
Yes, he lost a lot of men and ships at the Battle of Blackwater Bay, but he (Ser Davor) was able to convince the Iron Bank to give him money to hire more men and ships. In the most recent episode, they spoke of some mercenaries who deserted them due to the weather. Stannis mentioned something about them only valuing money.