The cost listed is the amount to acquire that size from the previous one.
(eg: if your island is 14x14 your next expansion will cost you 500,000 coins or 50 MOS; which is the "15x15" item on the list)
14x14 250,000coins / 25MOS
15x15 500,000coins / 50MOS
16x16 1,000,000coins / 100MOS
17x17 2,000,000coins / 200MOS
18x18 4,000,000coins / 400MOS
19x19 8,000,000coins / 800MOS
20x20 16,000,000coins / 1600MOS
21x21 20,000,000coins / 2000MOS
22x22 22,000,000coins / 2200MOS
23x23 24,000,000coins / 2400MOS
24x24 26,000,000coins / 2600MOS
25x25 28,000,000coins / 2800MOS