How Dicey are super cheap brake cables? I was going to buy some super cheap - I mean barely more than 50 cents per meter - brake cable.

Am I in any danger of sudden failure?  I don't mind replacing the cable after a year or something, but I'm sort of worried about it outright breaking and then trying to kill me.
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.