By my calculation, for 10A cable the copper is about 30 cents versus 15 cents for 2.5A.
Approximations: cable is 1 metre long, cable is three core, copper is NZD10 per kg, individual wire core cross sections are 1mm² versus 0.5mm² as per http://www.epanorama.net/documents/wiring/wire_resistance.htl
All those savings add up, the thing is in NZ the smaller cables all have to be 0.75mm² anyway, so there isnt the savings you would hope for over the 1mm² needed for a 10 amp cable.
Its about time the IEC did another slimmer plug IMO, my dell power supplies all have a bump on the end to accomodate the cloverleaf socket. if they had a figure 8 they could be thinner, but I guess they want to save a little bit by making it an earthed power supply vs double insulated. Also a 2 pin would let them use a smaller mains plug on the other end.