Build vs Buy Calculator
Compare the total cost of ownership of building in-house vs buying/subscribing over a chosen period, to decide which is cheaper.
輸入資料
計算結果
重點速覽：Build vs Buy compares the total cost of ownership of developing in-house vs buying/subscribing, over the same period. build total = development + annual maintenance x years; buy total = purchase + annual subscription x years; difference = build - buy (positive = buy cheaper, negative = build cheaper). Example: build 200k dev + 30k/yr; buy 50k + 40k/yr; 5y: build 350k, buy 250k, diff 100k (buy cheaper). WARNING: Only quantifiable direct costs; strategy, lock-in, talent and opportunity cost matter too; test multiple periods. For major calls, add professional advice.
計算公式
自建總成本 = 開發成本 + 每年維護成本 × 年期。
外購總成本 = 購置成本 + 每年訂閱成本 × 年期。
成本差額 = 自建總成本 − 外購總成本 (正值代表外購較省)。
$$C_{\text{build}} = D + M \times y$$$$C_{\text{buy}} = P + S \times y$$$$\Delta = C_{\text{build}} - C_{\text{buy}}$$使用說明
- Enter the build one-off development and annual maintenance.
- Enter the buy one-off purchase and annual subscription.
- Enter the comparison period to see both totals and the difference.
以自建 (開發 20 萬 + 每年維護 3 萬) vs 外購 (購置 5 萬 + 每年訂閱 4 萬) 為例，不同年期的總成本對照表
| 比較年期 | 自建總成本 | 外購總成本 | 成本差額 (自建−外購) | 何者較省 |
|---|---|---|---|---|
| 3 年 | HK$290,000 | HK$170,000 | +HK$120,000 | 外購較省 |
| 5 年 | HK$350,000 | HK$250,000 | +HK$100,000 | 外購較省 |
| 8 年 | HK$440,000 | HK$370,000 | +HK$70,000 | 外購較省 |
| 10 年 | HK$500,000 | HK$450,000 | +HK$50,000 | 外購較省 |
| 15 年 | HK$650,000 | HK$650,000 | HK$0 | 打和 (損益平衡) |
自建『前期重、後期輕』(開發 20 萬、維護僅 3 萬/年)，外購『前期輕、後期重』(購置 5 萬、訂閱 4 萬/年)。年期越長，外購的訂閱費不斷疊加，差額越縮小 — 到第 15 年 (損益平衡點) 兩者打和，之後自建反而更划算。損益平衡年 =(開發−購置)÷(訂閱−維護)=(200,000−50,000)÷(40,000−30,000)= 15 年。
理財情境案例
年期決定結論：3 年外購省 26 萬，8 年只差 6 萬
某公司要導入一套業務系統。自建:開發 500,000、每年維護 50,000;外購:購置 120,000、每年訂閱 90,000。先看 3 年:自建 = 500,000 + 50,000 × 3 = 650,000，外購 = 120,000 + 90,000 × 3 = 390,000，差額 = +260,000，外購大幅省 26 萬。
再看 8 年:自建 = 500,000 + 50,000 × 8 = 900,000，外購 = 120,000 + 90,000 × 8 = 840,000，差額縮小到只剩 +60,000。可見隨著年期拉長，外購每年 9 萬的訂閱費不斷累積，與自建的差距快速收窄。若這套系統預計要用 10 年以上，自建很可能反超。這個案例說明:『自建還是外購』的答案高度依賴你打算用多久 — 短期專案傾向外購 (初期投入低)，長期核心系統則要認真評估自建。切勿只用單一年期就下結論。(數字為便於說明的假設值。)
當自建更便宜：低訂閱門檻反而拉高外購成本
某能力自建成本不高:開發只需 150,000、每年維護 20,000;而外購雖然購置便宜 (30,000)，但每年訂閱高達 60,000。以 5 年計:自建 = 150,000 + 20,000 × 5 = 250,000，外購 = 30,000 + 60,000 × 5 = 330,000，差額 = −80,000 (負數)，即自建反而省 8 萬。
這個案例與『外購較省』的預設情境相反，原因在於外購的年訂閱費 (60,000) 遠高於自建的年維護 (20,000)，且開發成本本身不算高。當外購方案的『經常性費用』偏高、而自建的開發門檻又不高時，即使在中短年期，自建也可能勝出。這再次印證:不能想當然地認為『外購一定較便宜』或『自建一定較貴』— 一切取決於雙方實際的一次性成本與經常性成本結構。務必用自己真實的數字試算，並提醒:即使自建帳面較省，仍要評估內部是否有足夠人才與時間去開發及長期維護，否則隱性成本可能超出帳面。
常見問題
What does a positive or negative difference mean?
Difference = build total - buy total. Positive means build costs more, so buying is cheaper over that period (e.g. +HK$100,000 = buying saves ~HK$100,000). Negative means build is actually cheaper. Near zero means they are close, so decide on non-financial factors (control, flexibility, risk). The result is highly period-dependent: build is front-loaded (high dev, low upkeep); buy is light upfront but subscriptions accumulate. Longer horizons often favour build; short ones favour buy. Test 3, 5 and 8 years.
Is cost the only thing to consider?
Far from it. Cost is the easiest to quantify but not necessarily the most important. Beyond cost, weigh: build — full control of features and data, tailored to needs, possible competitive edge, no vendor lock-in; buy — faster rollout, vendor-maintained, low upfront, lets you focus on core business, risk shared with vendor. Also consider in-house talent and time, the opportunity cost of building, and whether the capability is core (build) or non-core (buy). Cost analysis is the start, not the end.
When can build actually be cheaper?
When the buy option's recurring fee is high but build's development hurdle is low. Example: build dev 150k + upkeep 20k/yr; buy purchase 30k but subscription 60k/yr; over 5y build = 250k, buy = 330k, difference = -80k — build is HK$80k cheaper. So never assume buy is always cheaper or build always dearer; it depends on the actual one-off and recurring structure. Also check you truly have the talent and time to build and maintain.
How do I read the break-even year?
The break-even (payback) year is where the two lines cross: break-even years = (development - purchase) / (subscription - maintenance). In the example (200k-50k)/(40k-30k) = 15 years. Before it, buy is cheaper; after it, build is cheaper. Use it to judge if you will use the capability long enough for build to win.
Any Hong Kong notes?
For Hong Kong SMEs, this supports decisions on IT systems, equipment and process build-outs; factor in Profits Tax deductibility of the costs and the Inland Revenue Ordinance's record-keeping (7 years). The HKPC and HKTDC offer SME support. Treat the result as a financial reference, not the sole basis.
相關工具
參考資料
內容審核：香港計算器財經團隊。總擁有成本 (TCO) 比較與損益平衡公式參考管理會計通用做法，並就本地企業數碼轉型情境參照香港生產力促進局及創新科技資助資訊，結果僅供教學與估算參考。