A way to remember it:
It cost the same number of points that the bonus of the current score, minimum of one.
13 to 14 cost one point
14 to 15 cost two points
16 to 17 cost three points
etc.
So for an 18 from an 8, it cost 1+1+1+1+1+1+2+2+3+3 = 16 points