The first cost for aluminum windings will be less than for copper.
If the transformer is for a critical application, you need to consider the issue of transformer repair. If a transformer with aluminum windings has an internal winding fault, the aluminum will leave an incredible splatter mess inside the tank. Copper will usually not be nearly so bad, making it more likely that the transformer can be repaired.
If you are an electric utility with 25 of these transformers in your bone yard, this may not be a factor, but if this transformer powers your manufacturing line, it could be important.
You may also want to base your evaluation on the life-cycle cost of the transformer losses. This is done by computing a present worth value to the future core and load losses of your transformer. This cost is then added to the purchase cost for purposes of evaluation. You will have to make assumptions regarding load factor and power costs. This helps to put everyone on an equal footing in terms of efficiency.