Steel wire is cheap (if it's not stainless). Even the outers aren't very sophisticated. So it's possible to meet the basic requirements for very little cost. For more money you get a lower friction lining and better tolerances, which are worth having in many cases (including beater bikes and BSOs with marginal brakes).
On the other hand if you need to keep taking up slack in the cable, something is wrong. I'd expect to set them up, ride a bit, tweak and then get hundreds of miles before needing to tighten again (unless your pads wear very fast). If that's not the case, something is creeping, stretching, fraying or otherwise letting you down.
After any work on the brakes, ride cautiously for a bit. Test for emergency stop rather than relying on it. For major work (like new cables or unfamiliar new pads) I like to do one brake and give it a good test before doing the other.