Land expansions
  Level Costs in island dollars Costs in gold
Expansion (Land) 5/25 10,000 50
  6/25 66,000 350
  7/25 82,000 900
  8/25 200,000 1,050
  9/25 360,000 1,250
  10/25 610,000 1,500
  11/25 900,000 1,800
  12/25 1,300,000 2,050
  13/25 1,800,000 2,500
  14/25 2,400,000 2,850
  15/25 ?,000,000 3,200
  16/25 4,200,000 3,500
  17/25 5,400,000 3,900
  18/25 ?,000,000 4,250
  19/25 ?,000,000 4,600
  20/25 ?,000,000 4,950
  21/25 14,000,000 5,300
  22/25 17,000,000 5,650
  23/25 20,000,000 6,000
  24/25 25,000,000 6,350
  25/25 30,000,000 6,700
   
  courtesy of Larino's Corner: spreadsheet

UPDATED 6/172019