That's a lot of copper. Have you considered a pair of transformers - boost line side (supply) from 240V(?) to 480V(?), then reduce back to 240V at the sub-panel - cutting most of your wire run's size requirement ~ in half? Maybe a wash on $$, but if you find the right deal of xfrmrs...??
...assuming code enforcement allows this. (I've seen it done, but no idea if it was suitably blessed by authority.)