abilityCost: {
    10: 0;
    11: 1;
    12: 2;
    13: 3;
    14: 5;
    15: 7;
    16: 10;
    17: 13;
    18: 17;
    7: -4;
    8: -2;
    9: -1;
} = ...

The point cost to increase an ability score using Point Buy