a new Goodyear Eagle costs average of £128 per corner and comes with max 7/8 mm tread . part worn 5mm for £25 . end of the day ya pays ya money ya take ya choice . next time you take a flight have a look at the remoulds they fit scarry
Aviation stuff is made to much higher specifications and tolerance

Most new tyres are 9mm. Virtually all tyres' grip level fall off a cliff at around 3mm, sometimes 4mm. Also, the more worn the tyre, the quicker it wears, quite dramatically on some tyres. The (lack of) grip is the reason why many choose to change tyres at around 5mm, which is why there are always loads of 5mm part-worns available, esp Michelin Pilot Sport, Pirelli P6000 and Avon ZV3/5.
So 5mm gives at most 2mm (often less) of quick wearing tread. For £25.
A new tyre gives 6mm of longer lasting tread. For £90 (the last set of Sport MAXX I had fitted this year).
So, over the life of a tyre, costs are probably in the same ballpark, only I get to choose the tyre I want, and know its history, without having to worry if it was off an accident damaged car, and what damage the tyre sustained in said accident. And I get the levels of grip that 1.7t, 200+ bhp of car deserves.
Cliche I know, but its the difference between spending more time on this Earth, or early trip to St Peter (or whatever religion you may or may not follow)