An API integration is a bridge between two systems that don’t talk automatically - for example, your webshop that needs to send orders to accounting software, or a CRM that needs to sync with email marketing. Typical cost: €800-€3,000 for a simple integration through tools like Zapier or Make, €3,000-€15,000 for a mid-complexity custom integration, and €15,000-€50,000+ for enterprise integrations with ERP systems or high data volume. Main factors: how different the systems are in their data, how much volume, and how reliable the communication must be.
This article breaks down what API integrations actually do, when to pick a ready tool vs custom, and why some “simple” integrations fail.
What an API integration is in plain language
An API (Application Programming Interface) is an agreed protocol for how two programs exchange data. Instead of a human retyping information from one system to another, the systems talk directly.
Typical example:
- A customer buys a product in your webshop
- The webshop sends a “new order” message to the accounting system through the API
- The accounting system generates an invoice, reduces stock, and notifies the warehouse
- All in 2 seconds, no human in the loop
API integration is exactly that: a programmatic “conversation” between two systems that saves time and prevents errors.
Most common integration types (and what they cost)
| Integration type | Cost (€) | Duration |
|---|---|---|
| Webshop → accounting (Pantheon, Minimax) | 1,500 - 5,000 | 1-3 weeks |
| CRM → email marketing (Mailchimp, Klaviyo) | 800 - 3,000 | 1-2 weeks |
| Mobile app → backend API | 3,000 - 10,000 | 2-4 weeks |
| Payment integration (Stripe, Corvus, Wspay) | 2,000 - 6,000 | 2-3 weeks |
| Shipping integration (GLS, DPD, Post) | 1,500 - 5,000 per carrier | 1-2 weeks |
| ERP integration (SAP, Pantheon, Microsoft Dynamics) | 8,000 - 30,000+ | 4-12 weeks |
| IoT / hardware → backend | 5,000 - 25,000 | 3-8 weeks |
| AI/LLM (Claude, GPT) integration into a business process | 3,000 - 20,000 | 2-8 weeks |
Prices are for a single integration. Multiple integrations usually get a discount (because the infrastructure work is shared).
Zapier/Make vs custom - when to pick what
Zapier and Make (Integromat) are off-the-shelf tools for simple integrations without coding. You set them up through a visual interface. Monthly subscription €20-€500.
When they pay off:
- Less than 1,000 events per month
- Integration is 1-to-1 (one trigger → one action)
- Systems with mutual support (both have a Zapier connector)
- You need a quick prototype or MVP
When they don’t work:
- Large data volume (above 10,000 events/month gets expensive)
- Complex logic (multiple conditions, data transformations)
- Systems without a ready connector
- You need low latency (Zapier typically has 1-15 minutes of delay)
- You need reliability guarantees and transparent error handling
Custom integration pays off for everything else. Higher upfront cost, but better long-term economics.
Typical decision example: integrating a webshop with 200 orders/month → Zapier (€30/month) can work. Webshop with 2,000 orders/month → Zapier costs €200+/month, custom (€3,000 one-time) pays back within a year.
What actually drives the cost
The six biggest factors:
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Whether both systems have an API. Most modern SaaS tools do. Old Windows apps from the 2000s often don’t, which means an expensive workaround.
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Quality of API documentation. Well-documented API → 50% faster development. Poorly documented API → the developer has to “discover” how the system works.
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Data volume. An integration transferring 100 rows daily is cheap. An integration transferring 100,000 rows daily requires careful architecture, batching, and monitoring.
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Data transformation. If data formats match, integration is simple. If one system uses “first + last name” and the other “full name,” you need code to map it.
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Direction of communication. One-way (only A to B) is simpler. Two-way (with conflict resolution) is 2-3x more complex.
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Error tolerance. Should the integration recover from network errors on its own? Should it alert if something fails? Should it run a parallel backup path? Everything adds scope.
What often fails - and how to avoid it
The five most common reasons API integrations “don’t work”:
1. No error plan. What happens when the target system doesn’t respond? Does the integration retry, notify the user, or silently lose data? Without a plan - silent loss.
2. No monitoring. The integration runs for 6 months, then one day stops. Who knows? No one, until a client calls a week later. Monitoring is mandatory.
3. Wrong approach for volume. An integration designed for 100 orders/month falls over when there are 10,000. The volume conversation has to happen in Discovery.
4. The other system’s API changed. An external system (Stripe, some external CRM) makes an API change, integration stops working. A real development partner tracks the API changelog of its dependencies.
5. No maintenance plan. Integrations, like all software, need maintenance. Typically 10-20% of initial cost per year.
A real integration development flow
What we actually do:
Week 1: Discovery
- Mapping both systems and the data flow
- Reviewing API documentation
- Estimating volume and frequency
- Defining what “successful” integration means
Week 2-3: Development
- Setting up development environment
- Writing integration code
- Mapping fields between systems
- Error handling and edge cases
Week 3-4: Testing
- Testing with real data
- Stress test (how it behaves with lots of transactions)
- Error test (what happens when the target system doesn’t respond)
Week 4-5: Production
- Deploying to production (with monitoring)
- Running in parallel with the old way for 1-2 weeks
- Gradually shutting down the old way
Total: 3-8 weeks for a standard mid-complexity integration.
Frequently asked questions
Can we use Zapier in production with thousands of transactions per month? Technically yes, but it gets expensive (€500+/month) and fragile. Above 1,000-2,000 events per month, custom is usually the more rational choice.
Who’s responsible when an external system changes its API and our integration stops working? In a good development contract, that’s part of maintenance - we track and update. Without a maintenance contract, you’re on your own when something breaks.
Can we get an integration for €500? Realistically - only if you use Zapier and both systems are off-the-shelf. Custom integrations start around €1,500 for the simplest cases. Lower prices are usually a sign someone doesn’t understand how long it actually takes.
How do we know the integration is “done”? Three criteria: (1) it reliably transfers data in both directions (if two-way), (2) it has monitoring that alerts us if something breaks, (3) it’s documented so someone else can take over if we step back.
Need an integration?
Book a free Discovery call. We review your systems, propose the integration architecture, and give a concrete price - not a “somewhere between €X and €Y” range.
Reach out at [email protected] or through the form on our homepage.