The Extended Point Buy system uses a cubic regression formula to set the costs for ability scores. This formula follows the pattern of the standard costs but extends them to cover both lower and higher scores. It ensures that scores above 15 become increasingly expensive, while those below 8 get cheaper.

By default, this system allows for negative score costs. This option lets players build characters with significant weaknesses in some abilities to gain exceptional strengths in others, creating more diverse characters. However, this might lead to an imbalance in the game.

You, as the Dungeon Master, have the flexibility to modify these settings to better fit your campaign. If you want to avoid potentially unbalanced characters, you can turn off the ‘Allow Negative Score Costs’ option. This will prevent ability scores from having a negative cost.

Keep in mind that there are no official rules for this kind of point buy extension. Nonetheless, it’s a versatile solution for those looking to add depth to character creation. Feel free to adjust the cost as needed for your game.