First off, the depth and total amount of gallons are a good start.
Several factors to consider:
1) the Earth has a natural temp of 56-57F. That changes along some portions of the surface, e.g., temps go lower in the top-soil layer and soil can freeze due to ambient air temps. (This is the so-called the 'freeze line'.) Underneath the pond, unless the freeze line goes very deep, the ground will still be 56-57F.
2) wind and ambient air temps will strip heat faster than anything, so deeper ponds with wind breaks are better than open air shallow ponds
3) certain fish handle low temps a lot better (naturally living in temps 55-65), and in large ponds that freeze over from time to time there will be many who last the winter if the pond is not too shallow
4) should you look exclusively at tropicals (especially cichlids) ones that handle lower temps are preferable
Overall, insulate well (especially at depths down to the freeze line), keep 'most' of the pond below the surface and block off wind. Temporary winter structures (tent's for example) might provide a lot of savings if needed. Obviously, your particular part of the world has it's own weather patterns that will affect this, as well as the particular topography of where the pond is place.
