Should have, could have, would have. You cant question his decisions. He did what he did under the circumstance and under the influence of his character.
Ethan was, by no doubt, a poor man. He had a strong sense of responsibility and duty first seen abondoning his studies to care for his father. Then after his mother died, he married Zeena out of duty for caring for his mom and he didnt wanna be alone.
He now feels guilty that if he leaves she will be'alone and destitute' Further more his pen'paused on the word money' because he wouldnt have enough money to even make it west with Mattie.
You might argue that he had a chance to take money from the Hales but again his sense of honesty and pride didnt allow him to trick the hales for money.