It just seems strange that the Persians would trust Greek mercenaries to be elite, loyal troops when a "mercenary" can in theory be bought quite easily by someone wealthier, as Alexander eventually came to be. Yet according to Richard Freeman's biography, these Greek mercenaries fought for Persia until the end.
To answer your question, I think it's best to understand that not all Greek Mercenaries were the same, although it's fair to say regardless of if they were rowers or hoplites that money was the primary concern regardless of if you were working for the Athenians or the Persians.
There's actually a large sum of documentation from Commanders and those like Xenophon that speak of the supply-demand system that was in place. Basically, each army had to pay the best, to get the best (which makes sense). It also wasn't unheard of to sign contracts that would guarantee a ration of food along with your pay, which would, to an extent prevent you from deserting or betraying your employer (honor was definitely a big deal when it came to these people, they weren't like the characters you see in movies that will betray whoever for their cut of the reward)
It's rather important to take into consideration the motivations behind becoming a mercenary, obviously many had military training, and many sought wealth through what they perceived as the best way (they weren't inheriting wealth by any means). In essence, whether it was a large monthly wage, or the promise of land and power; people often chose to align themselves with the highest bidder.
To be fair, this isn't always the case, Trundle's book on Greek Mercenaries mentions Xenophon stating "Prince Cyrus did so not from need, but from a belief in Cyrus’ good qualities and his arete or nobility (Xen. An. 6.4.8; Parke 1933: 29; Roy 1967: 319)" so it is fair to assume that although the reward was the immediate inspiration for joining a cause, mercenaries also had other things influencing their choice of employer.
Here's a link to the online copy of the book I mentioned, it basically covers any and all questions regarding the topic of Greek Mercenaries:
Forgot to post this.
Besides what was already said, Alexander had just razed Thebes to the ground (except for Pindar's house) so its likely there were a lot of anti-Macedonian Thebans in the Persian forces.