Your Real Break-Even Price Isn't What You Paid for Your First Share
6 min read
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6 min read
More than 30 million people in Indonesia now hold a capital markets account, and almost all of them, 99.8%, are individual retail investors, not institutions. A lot of them do the same thing you probably do: buy a stock, watch the price move, and buy more of it later at a different price.
Once you have bought the same stock more than once, "what did I pay for this" stops having one answer. Your portfolio app blends every purchase into a single number, your average cost basis. But that blended number is not quite the price you need the stock to hit before you are actually even. The real one sits a little higher, and the gap is easy to miss.
Average cost basis is the total amount you have spent buying a stock, divided by the total number of shares you bought. It is a weighted average: a purchase of 300 shares counts three times as much toward the average as a purchase of 100 shares, even if the 100-share purchase happened first.
Average cost basis = (Σ shares bought × price paid) ÷ Σ shares bought
This is also exactly what happens inside a dividend reinvestment plan. Every time a reinvested dividend buys you a few more shares at whatever the price happens to be that day, it pulls your average cost basis the same way a manual purchase would. See the full mechanics of that in how dividend reinvestment compounds faster than spending the payout.
Say you buy a stock in three separate lots over a few months, each at a different price:
| Purchase | Lots (1 lot = 100 shares) | Shares | Price per share | Cost |
|---|---|---|---|---|
| 1 | 1 | 100 | Rp3,000 | Rp300,000 |
| 2 | 3 | 300 | Rp2,400 | Rp720,000 |
| 3 | 2 | 200 | Rp2,800 | Rp560,000 |
| Total | 6 | 600 | avg Rp2,633 | Rp1,580,000 |
Add up the cost column, Rp1,580,000, and divide by the total shares, 600. Your average cost basis is Rp2,633 per share. Not Rp3,000, what you paid the first time. Not Rp2,400, the lowest price you bought at. A blend of all three, weighted by how many shares each purchase added.
If the stock is trading at Rp2,700 today, comparing it only to your very first purchase price makes it look like a 10% loss. Compared to your actual average cost basis of Rp2,633, you are up about 2.5%. Same stock, same day, two very different answers, because the first one ignores that you bought more later at a lower price.
Rp2,633 is still not the exact price you need to walk away with nothing gained and nothing lost. It leaves out one thing: buying and selling both cost money.
Indonesian brokers charge a fee on every trade. Mandiri Sekuritas, one of the country's larger brokers, publishes an all-in buying fee of about 0.18% of the transaction value and an all-in selling fee of about 0.28%, a figure that already bundles the broker's own commission, the exchange levy and the 0.1% final income tax the government charges on every share sale. Other brokers vary, but a gap between the buy-side and sell-side cost is standard across the industry.
Add the 0.18% buying cost to the Rp1,580,000 spent above, and the true cost of building the position is closer to Rp1,582,844. Then, because selling will cost another 0.28% off whatever price you sell at, the price you need to actually break even has to clear that cost too.
True break-even price = (Total cost × (1 + buy fee %)) ÷ (Shares × (1 − sell fee %))
Working through the numbers: Rp1,582,844 divided by 600 shares adjusted for the 0.28% sell fee comes out to about Rp2,645. Compared to Rp2,633, that is roughly Rp12 more per share, about 0.5%, small on any single trade but real, and it compounds every time you buy and sell.
Every new purchase moves your average cost basis in the direction of the price you just paid, pulled toward it in proportion to how many shares you bought.
Buy more at a price below your current average, and your average cost basis drops. This is the mechanical fact behind "averaging down." Buy more at a price above your current average, and it rises instead, sometimes called averaging up.
Open your Holdings view in NetWort and look at the Cost Basis figure next to each position. That number is calculated the same way as the worked example above: every buy transaction you have logged, weighted by quantity, blended into one average.
It is only as accurate as what you entered. If you logged the raw price per share without folding in your broker's buying fee, your actual break-even sits a little above what NetWort shows, the same gap this article just walked through. If you ever plan to sell foreign shares rather than IDX-listed ones, that same cost basis number also decides your tax bill, not just whether you are currently up or down, and FIFO versus average cost can change what you owe.