He should buy Greenland, there are so many upsides to a purchase:
1. Military and commercial control of the arctic
2. Resource development/mining and drilling
3. Real-estate development
4. Retail business
5. Transport industry
6. Fishing industry
Mr Trump would like a new golf course and hotel in exotic places, so this would fit point 3. Also with weather patterns changing, Greenland might actually become green (rather than white) so its like a long term investment. Perhaps it would become associated state like Puerto Rico, no voting in congress.