Remember the old StarTrek episodes where the computer would go klicky-klack for a couple of seconds and then intone "Insufficient Data for Meaningful Response"?
Copper versus Aluminium is not the issue. The issue is initial cost versus efficiency and operating cost. You need to get the specifications on the loss in the transformer at different load levels, and you need to calculate the loading that you expect on the transformer. You combine the expected loading and the loss data to determine the losses to be expected over the operating time of the transformer. You should also include the cost of air-conditioning if the transformer is in a cooled space.
The Copper Development Association has lots of resources for doing these calculations, but they are a shill for the copper industry, and will have you believe that a Copper wound transformer is inherently more efficient than an Aluminium wound transformer. This is not necessarily the case, and it is really necessary to get the specs from the transformer manufacturer.
Very often it pays to buy a more efficient transformer, even with a considerable price premium. Those losses keep ticking 24x7x365, so reducing the losses means reduced electrical power consumption.