https://www.quora.com/What-is-the-relat ... -of-a-lens
power = 1 / focal_length = (refractive_index - 1) / radius_of_curvature
(power is in diopters, and focal_length and radius_of_curvature are in metres.)
Specsavers does not say what the refractive indices of its lenses are, but I found this for Superdrug:
https://opticians.superdrug.com/which-lens-thickness
Bronze Standard 1.5 index CR39
Gold 1.6 index
Gold+ 1.67 index
Platinum 1.74 index
Let R = radius of curvature, r = radius of the lens and x = the thickness of a single curved surface lens that is infinitely thin at the centre.
For a concave lens (I am short sighted), Pythagoras's Theorem gives:
(R - x)^2 + r^2 = R^2
R - x = sqrt(R^2 - r^2)
x = R - sqrt(R^2 - r^2)
My computer glasses would need a power of -6 diopters. Plugging the numbers into a spreadsheet gives:
Bronze 1.5 3.2 mm
Gold 1.6 2.7 mm
Gold+ 1.67 2.4 mm
Platinum 1.74 2.2 mm
That is 1 mm difference in thickness between the cheapest and the most expensive lens. I also have some astigmatism, which would add the the thickness of the lenses (and the centre of the lens would not be infinitely thin either). Nonetheless, the improvement given by the expensive lenses would not be much to shout about.