Once we factor in ongoing maintenance (not just the initial build), does a custom Shopify Function ever actually beat a third-party discount app's subscription cost over 2-3 years?

What do a custom Function and a discount app cost over three years?

Deploi's Scripts-to-Functions record puts an illustrative $7,250 to $40,000 on three years of a custom Shopify Function, code Shopify runs at checkout, upkeep included. Its illustrative app path costs about $1,800 to $16,700 over the same three years (Deploi Build vs. Buy, October 2026). Who builds it is the in-house or agency staffing decision.

One common rule costs far less on an app. Dealeasy, a volume discount app, is free up to 50 orders a month, then $9 to $29 USD a month as orders rise (Dealeasy listing, October 2026). At $29 a month, three years cost $1,044 (Deploi arithmetic), so no custom build competes for that rule. A new discount Function starts higher than a port: see what an agency charges to build one.

When does a custom Function overtake the app subscriptions?

A custom Shopify Function overtakes the app path in four cases, and dropping one subscription is rarely one of them.

1. One rule set needs several subscriptions. Deploi's Scripts-to-Functions record, which prices porting old Shopify Scripts, warns that "One Script often becomes two or three subscriptions." Such stacks "close the gap around year three" (Deploi Build vs. Buy, October 2026). One no-code Function builder can hold several rules instead: SMART Functions Creator's Pro plan runs unlimited active functions for $39.99 USD a month (SMART Functions listing, October 2026).
2. The app prices on the orders it discounts, and you are growing. Regios Automatic Discounts runs from $34.99 a month for up to 500 discounted orders to $249.99 a month for unlimited orders, in USD (Regios listing, October 2026). Three years on the top plan cost $7,500 to $9,000 (Deploi arithmetic), level with the record's cheapest build.
3. The app cannot express the logic. Deploi's Scripts-to-Functions record warns that "Preset builders flatten stacked logic, and the near-miss shows up as margin leakage, not an error message." That leak, not the subscription, can exceed the build cost, because it lands on every order the app gets wrong.
4. The rule is a competitive asset. In our judgment, pricing logic nobody else can copy is not a cost line at all.

What does a custom Function cost to maintain after launch?

A custom Shopify Function costs about 15 to 20% of its build cost a year to maintain in Deploi's Scripts-to-Functions record, before any alerting (Deploi Build vs. Buy, October 2026). Two cost lines make up the bill:

• Upkeep. The record's illustrative share covers API (application programming interface) version upgrades and re-testing when promotions change, so it recurs every year the Function runs. Values hardcoded into a Function add a deploy to every routine change, as a shipping Function that needs a deploy for every new location shows.
• Alerting. Shopify's monitoring guide describes no merchant-facing alert (Shopify monitoring guide, October 2026), and Deploi's observability record found no App Store app that monitors Function runs. The record puts alerting at an illustrative $12,000 to $40,000 in year zero and $5,000 to $12,000 a year. That band covers two or three live Functions, so one Function costs less (October 2026).

What does a discount app cost beyond its subscription?

A discount app costs more than its subscription, because the vendor holds your rule and your exit path.

• Vendor risk on a pricing path. In our judgment, an app that changes its pricing model or gets acquired turns into a checkout migration.
• Configuration you do not own. A packaged app's campaign settings live in the vendor's admin, and "the discounts a packaged app created stop when the app is removed" (Deploi Build vs. Buy, October 2026).
• An exit you cannot export. Deploi's Scripts-to-Functions record warns that app-side rules leave as screenshots, and "re-porting later means re-specifying from scratch" (Deploi Build vs. Buy, October 2026).

When should you not build a custom Function to save money?