EDIT: Here is the list I am putting together of the hidden cost for expansions once you have unearthed (pun intended) the mine. Because they get VERY expensive, I thought people who scrimped and saved and bombed to get expansions might find it helpful to know which have more or less "Hidden Costs" than the others to make actually usable.
Thanks to igames4me for the image I used to make this chart. Following are all the "Hidden Costs" (in coins) for expansion:
Attachment 14636
Island 1, after expanding to the mine:
- Sinkstones: 18,000
- Siphoning Cypress: 30,000
- Toxic Sycamores: 45,000
- Poisonous Pumice: 90,00
- Wicked Thicket: 100,000
- Immovable Rock 180,000
Island 2:
1) Various obstacles totaling 27,500
2) Various obstacles totaling 55,000
3) Various obstacles totaling 80,000
4) Radiating Rock 100,000
5) Unlock electric animals and habitats, receive "free" Lightning Leopard and Electric Enclave on that square.
6) Burdening Birches (25:00): 90,000
7) 1,250 + 2,000 + 25,000 obstacles + Radiating Rock (100,000 2x2 and 21:40) = 128,250
8) 100,000 rock + 250,000 Fail Shale (33:20) = 350,000
9) Harmful Hemlocks (33:20): 450,000
10) 2 Fail Shales = 500,000
11) 2 Fail Shales + 1 Radiating Rock. Total = 600,000
12) 1 Fail Shale + 1 Harmful Hemlocks. Total = 700,000
13) 1 Frozen Fir (25,000) + 3 Fail Shales (250,000). Total = 775,000.
Expanding to 13 opens the first square of Island 3 (A on my legend map).
Island 3:
A) 1 Radiating Rock (100,000 2x2) + 3 Fail Shales (250,000). Total = 850,000
B) 1 Harmful Hemlock + at least 2 Fail Shales. Total = at least 950,000
C) 4 Fail Shales (250,000 each) + assorted shrubbery = a little over 1,000,000
D) 2 Radiating Rocks + 4 Fail Shales. Total = 1,200,000.
E) 5 Fail Shales + 3 Frozen Firs. Total = 1,325,000.
F) 6 Fail Shales + 2 mushy bushes + 1 frozen fir. Total = 1,527,500.
G) 3 Fail Shales + 1 Harmful Hemlock. Total = 1,200,000.
H) 4 Fail Shales + 2 Frozen Firs + 2 Seeping Cedars. Total = 1,055,000.
To expand the cheapest to most expensive on Island 3, use this order: A - B - C - H - DorG - DorG - E - F... pending confirmation B is in fact < C.
[NOTE: I Revised the island 2&3 map legend to make it more comprehensible. On island 2, the expansions are numbered in order from cheapest path to the leopard, then cheapest to most expensive of the rest.]
Thanks to SANDScape, Anyuszko, Petasos and cquinn32 for info on the expansions beyond the lightning leopard square.
I will continue to update (and correct if necessary!) as folks provide more info.