From the in-game manual (p.15):
  2nd -  5th Levels  5,000 Pts/level
  6th - 10th Levels 12,000 Pts/level
 11th - 15th Levels 36,000 Pts/level
 16th - 20th Levels 72,000 Pts/level

Which translates to:
 1st:       0
 2nd:   5,000
 3rd:  10,000
 4th:  15,000
 5th:  20,000
 6th:  32,000
 7th:  44,000
 8th:  56,000
 9th:  68,000
10th:  80,000
11th: 116,000
12th: 152,000
13th: 188,000
14th: 224,000
15th: 260,000
16th: 332,000
17th: 404,000
18th: 476,000
19th: 548,000
20th: 620,000