What is (roughly) the ratio of insulation-to-conductor cost in a cable?
Cross-sectional area will vary with square of diameter. Circumference will vary linearly.

So to answer the part related to Why aren’t the main conductors in this underground power cable made from copper?: 


Aluminum has 61 percent of the conductivity of copper. For the same resistance then we need \$ \frac {1}{0.61} = 1.64 \$ times the cross-sectional area.
The diameter of the aluminium cable will be increased by \$ \sqrt {1.64} = 1.28 \$ times that of the copper conductor.


If the diameter of the cable is large relative to the thickness of the insulation that would give a 28% increase in insulation cost.

According to Plasticker.de PVC is about €0.40 per kg. (I'm not in the business so I don't know what quality or plasticisers are required for insulation.) 


Aluminium seems to be about €1.78 / kg. Infomine.
Copper is about €5.80 / kg.