Some people say cables make no difference to the sound (assuming they are a basic good construction).
When I hear different cables I don't understand how they can say that.
However I will never be able to hear what they hear. To say they don't know what they are talking about is unfair. Equally for someone to tell me it's all in my head is unfair.
We all have to accept that some can hear a difference and some can't. If you can't maybe you're lucky as you can save some money for other improvements.
Now another argument is that the cost of a given degree of improvement is lower by spending it on a speaker upgrade than a cable. I'm not going to argue with that, it depends on your system. If you've got a £10k pair of speakers then £30/m cable will make them sound better than £5/m cable (assuming they're both a decent design). If you've got the £5/m cable you'll probably get a better improvement from the cables than upgrading to a £10,250 pair of speakers.
However if you've got a £250 pair of speakers you'll be better off buying a £500 pair of speakers than upgrading to the £30/m cables.
It's not rocket science.