Consider the values attached to letters in Scrabble (and frequencies):

1 point: E ×12, A ×9, I ×9, O ×8, N ×6, R ×6, T ×6, L ×4, S ×4, U ×4
2 points: D ×4, G ×3
3 points: B ×2, C ×2, M ×2, P ×2
4 points: F ×2, H ×2, V ×2, W ×2, Y ×2
5 points: K ×1
8 points: J ×1, X ×1
10 points: Q ×1, Z ×1

Pretty close, rank-wise, to the ordering in the graph above on English corpus letters.

I'm not sure how the Scrabble values/frequencies were derived originally - I guess I should look it up.