Little math with the help of the
calculator. If this option would halve the price, fair upgrade times would be...
Level 1: 4 minutes / wall or less
Level 2: 10 minutes / wall
Level 3: 26 minutes / wall
Level 4: 45 minutes / wall
Level 5: 2 hours 9 minutes / wall
Level 6: ~4 hours / wall (3 hours 58 minutes to be precise)
Level 7: 11 hours 9 minutes 30 seconds / wall
Level 8: 18 hours 44 minutes / wall
Level 9: ~ 1 day 15 hours / wall (1 day 14 hours 59 minutes to be precise)
Level 10: 4 days 20 hours 14 minutes / wall
Level 11: 5 days 22 hours 19 minutes / wall
Under this option, 5 builders could grind all walls to level 11 in... 296 days.