(n - r) * 10
Although this won't result in 1st place in a 30-player tournament getting 300 points. They'll get 290. That's because 30th place gets 0 points. If you want 1st to get 300 points, use:
(n - r + 1) * 10
Easy to see with a 3 player tournament instead of 30. Using first example, (n - r) * 10:
1st: 20
2nd: 10
3rd: 0
Using (n - r + 1) * 10:
1st: 30
2nd: 20
3rd: 10