Average cost basis

Your average cost is the total you have paid for the shares you hold, divided by how many you hold.

Buying more re-averages it. Buy 10 shares at $100, then 10 more at $200, and your average cost becomes $150 across all 20.

Selling does not change your average cost — a common surprise. The shares that remain keep the same per-share basis; the gap between what you sold at and your average cost is your realised gain or loss.

This is a simplification. Real accounts can track individual lots (FIFO and other methods), which can change the numbers at tax time. Aureus uses average cost because it is the one figure that stays easy to hold in your head.