It's all digital these days. The signals in the cables go through or they don't. Get the cheapest cable that will do the job. All the gold plating, silver cores, and whatever else they have is not going to magically improve your signal.
This isn't really true for Ethernet cables. If you're just going from a router to your computer in the same room, any cable will do. But if you're putting several Ethernet cables together and running them through a building, you'll want quality cables.
They won't just 'stop working' - they'll start producing bad data (which is corrected via tcp/ip but your throughput will suffer). I know this from experience - we are about to re-wire our entire building because the original contractor used crappy cat 5 cables and it is causing us all sorts of problems.
Edit: Count me in for monoprice.com. I've used them plenty of times in the past. Their HDMI cables are soo cheap it is ridiculous. I really like their cat 6a cables because of the metal connectors on the ends. My cats have bitten through the plastic ends on lesser cables and destroyed them