I was so surprised with that info (I supposed it is 2x) that I have checked at www calculator of the PCB manufacturer we use. I think it is the biggest (not the chippest) one in Poland and continuously improving his technology. Now they offer PCB with 4/4mils. But I have checked prices for their worst technology = 6/6mils.
Their 5 days offer (you have to order at least 1m²). So I have checked prices (netto) for 100PCBs 10x10cm (1,55mm width).
2 layer - 1784 PLN (about $465)
4 layer - 3783 PLN (about $985)
Their prototype offer - I have checked the price for 3PCBs 5x5cm:
2 layer - 108,75 PLN ($28)
4 layer - 195 PLN ($51)
I didn’t checked their standard production offer (if you order less then 1m²). It is more expensive and slower so if we order PCBs ourself we try to order at least 1m².