Increasing Ability Scores The following table provides a conversion from points to percentages:
 
Ability Score
Percentage bonus per point
1-5 
+20%/point
6-15
+10%/point
16-17
+5%/point
18-18/75
+1%/point
18/76-18/00
+0.5%/point
19-25
+0.1%/point

Once a stat increases past 100%, the character's score increases to the next higher number.
As you can see, the higher a stat, the harder it is to increase.
Stats CANNOT be increased past racial maximums, unless from a magical or godlike source.