As someone who grew up very poor in rural America in the 60s-70-s, I can say a few things. My dad worked construction, and made decent money, but it wasn't consistent. Remember at the end of the movie Ennis struggles with the decision whether to go on the roundup or go to his daughter's wedding.That sort of thing is common among people who work on an "as needed" basis.
Also, rural economic standards always lag behind more populated areas. I remember working as a dishwasher when I was sixteen for $1.50 an hour. (The minimum wage at the time was 3 or 3.15). Many many people held jobs working below minimum, and no one cared anything about it. I wouldn't be at all surprised if Alma's job paid below minimum. Likewise, my dad was paid less than he would have been paid in the city or the suburbs, though above the minimum.
When you work outside in a rural area, your work time is often curtailed by weather. Let's say they were going to castrate a bunch of calves and a huge rainstorm came up. Rather than try to round up the calves in the mud, they might wait a day or two... and the workers not get paid. I remember (as an adult) when I had a job working for the city government and they sent us home one day because of a snowstorm. When I told my dad about it, he asked, "You still get paid?" Of course I did, but he had to ask to be sure.
Also, people who haven't grown up with money and without parents who are good at managing money (or no parents at all, like Ennis) haven't learned how to manage their own finances. They might be careless with paying bills and end up with late fees, etc. They might not instinctively know how to make the best choices. And, as pointed out upthread, cigarettes and beer--always more popular among the poor--certainly eat up the funds quickly.
There were and are millions of people in America living in rural poverty. Ennis and Alma are, as I see it, a realistic example.