from an earlier post:
Please note, I say "the cost of an hour", not "hourly salary". The value of an hour has many components in it:
hourly wage
hourly benefits
hourly cost of rent, heat, AC etc
and more
the the most often forgotten:
the cost of missed business because resources were otherwise engaged.

Kai:
if I remember that in many countries situation in terms of fabrication of PCBs might not be as nice as in the USA. Multilayer boards can be very very costly at certain countries in the world.

Ok, lets calculate:
with the above let us set Prahads time to $50
let us set the added cost for 4 layers at $50

With that easy caluclation you see that for every hour Prahad spend making the 2 layer board acceptable, he need to make one more unit from the design.

If his predicted volume is less than the hours spent making 2 layers work he need to increase the price because of using 2 layers instead of 4

Erik