There seems to be no predictable cost to base expansions. I've filled out as much as I have documented.
Expansion times seems to be capped at 48 hours but cost has not.
Feel free to add to this thread and check postings for accuracy.
I will update original post whenever possible.
Any new players or those starting to expand base should add to this thread for a complete expansion cost from the 1st square (over and above the original 4) onwards.
Expand base squares:
34 - 3,100,000
35 - 3,300,000
36 - 3,600,000
37 - 4,000,000
38 - 4,300,000
39 - 4,600,000
40 - 5,000,000
41 - 8,000,000
42 - 8,500,000
43 - 9,000,000
44 - 9,500,000
45 - 10,000,000
46 - 11.000,000
47 - 12,000,000
48 - 13,000,000
49 - 14,000,000
50 - 15,000,000
51 - 16,000,000
52 - 17,000,000
53 - 18,000,000
54 - 19,000,000
55 - 20,000,000
56 - 22,000,000
57 - 24,000,000
58 - 26,000,000
59 - 28,000,000
60 - 30,000,000
61 - 32,000,000
Special thanks to GIJOE for latest updates.