But there will be a difference in the connectors on a £70 ones to 70p ones for sure. Gold casing etc. Which will make a difference.
Only in terms of it lasting longer and being less likely to suffer any drop out, if you get a 'good' cheap one (and most are) you are hugely unlikely to suffer any image quality issues. A very small minority of people suffer from interference with different equipment in the vicinity, in which case a better shielded cable will probably help, but even so, £30 is probably all the money you would ever need to spend.
It saddens me (but doesn't surprise me) that looking around in Currys last weekend, the cheapest HDMI lead they sold was £30, the most expensive being £80. Considering most Blu Ray players do not come with a lead, presumably most people are paying these prices.
I bought two from Amazon, each just under £2. One is very thick and feels really well constructed, the other feels a little more flimsy (but still connects just fine), it's the luck of the draw, but as they sit there and never move it's very unlikely that one is going to perish any time soon.