How cheap is McDonald's considered to be in the US? I don't think it's fair to compare a McDonald's to an "expensive burger", just as you wouldn't compare an entry-level cheap car to a Rolls-Royce. Yeah, they're both "burgers", but that's about it.
I also think GP's point was a bit different, as in eating out altogether may have been seen a treat, not McDonald's specifically. Eating in was more the norm, for price reasons.
Over here in France, I do consider it expensive for what it is. I can easily whip together food that is healthier, tastier and cheaper.
And when I say "easy" I also consider the time factor. It takes me less time to chop up some vegetables, throw them in a pot over some minced meat and do the dishes while it cooks. I can also cook some rice / pasta / whatever to go with it. It usually takes around 30 minutes from start to finish. Unless the McDonald's is right next door, it won't even be faster.
It's been years since I haven't been in a McDonald's, but a menu used to cost some 7-8 €. Most other fast-food places (think kebab joint) used to cost roughly in the same ballpark. The kind of recipe I'm talking about above will usually come in a bit cheaper, and there are multiple servings.
Of course, there are cases when you're out and about and can't cook, so YMMV, but for many places where people work "inside" (as opposed to, say, construction), there usually are microwave ovens available, so you can reheat your own food.