it's, you start out with 8 in each ability score
Then, you use up one point per ability point gained, up to 14.
After that, the cost gets higher.
so
score points
8 0
9 1
10 2
11 3
12 4
13 5
14 6
and I think
15 8
16 10
17 13
18 16
That's my off-hand thinking, I'm sure 16=10 and 18=16, but I'm not sure about 15 and 17