How far was your commute? If it's like 2-3 miles, yeah, you should be fine, because you could just walk when something breaks. If it's 5-10 miles, that's a lot less feasible.

My commute went through some farmland for a few miles, far from the bus lines, so having something more reliable was more important. I spent about $500 and put thousands of miles on it over the first few years of ownership. The only issues I had were flat tires, so I put on denser tires and that went away (Schwalbe Marathon Plus). I would average 15-20mph, depending on wind, and I don't race or anything like that.

A big box bike might work okay for a while, or maybe the person who put it together did it wrong. Even if it's put together right, they use crappy components so it'll be a giant pain to ride vs a better bike. They're heavy, have lots of friction in the drive train, the tires are crap, and shifting often breaks after a year or so.

Even with a $500+ bike, you'll save money, and probably on your first year if it's replacing a car or bus. Do yourself a favor and get a lower end bike from a quality brand.