I'll stick to pound shop quality!
The argument that "10110" is "10110" is only part of the story. There's slew rate, ringing, over-shoot and under-shoot.
Why do digital signals become unusable at some particular distance? Look up 'capacitance per metre' of cables. Now imagine a length that works but with a different capacitance. It's simple really.
Now look up 'Transmission line' -
Capacitance per metre is closely linked with characteristic impedance; it does not cause distortion of the signal.
Slew-rate is primarily a characteristic of the Driver, not the cable.
Ringing and overshoot occur with un-terminated connections, not propertly matched transmission lines.
The ONLY relevent characteristic of a balanced digital transmission line is the attenuation; the signal at the receiver must be a suitable amount above the receiver threshold levels.
As others have said, dirt cheap CAT5e cable is rated for 100m per leg with each pair rated to 350 MHz. Cost of the cable itself is in no way a major factor in the selling price
The attenuation of any normal HDMI cable is trivial, over those distances any moderate quality paired cable should work.
Any argument that cable quality can cause subtle changes on a digital link are ludicrous.
If anyone is in doubt - try very slowly pulling an optical cable out of it's socket while you are playing music through it and see what happens when a digital signal degrades below the receiver threshold.
It is in no way subtle, so watch your speakers & eardrums!