I run a small open project that tracks what B2B software integrations actually cost — not the sticker price, but the true monthly cost once you factor in plan-tier gates, API add-ons, and middleware fees.

Three findings that surprised me

1. Salesforce charges $25/user/month EXTRA for API access on Professional ($100/user). A 40-person team pays $12,000/year just for the right to connect their tools. This is buried in Salesforce's developer docs and never mentioned on their integration pages. I documented the full trap here: https://stackcostmatrix.com/stack/salesforce-and-slack/

2. HubSpot's Starter plan is $20/seat — but Breeze AI in Slack, advanced workflow notifications, and record-creation shortcuts all require Professional or above. The "free" Slack integration quietly forces a tier jump. Full breakdown: https://stackcostmatrix.com/stack/hubspot-and-slack/

3. The native-vs-middleware math flips at team size. A $30/month Make.com subscription often beats a $20→$100/seat tier jump. Nobody shows this math — so I built calculators that do, with a full TCO engine (total stack cost, incremental integration cost, first-year total, and 3-year TCO): https://stackcostmatrix.com/crm-integration-pricing-guide/

The open index

I put all of it into a free, open index — 55 integration pairs, each with an interactive true-cost calculator (team-size slider, required-tier toggle, "I already pay for both apps" incremental mode), a hidden-cost trap breakdown, and the exact minimum plan tier that unlocks each connection.

Methodology

Every price is re-verified against live vendor pricing pages and developer API docs (all 40 source URLs are linked and timestamped on the About page). If a vendor changes a tier, the mismatch gets flagged for review — never silently rewritten.

Curious — what's the worst hidden integration cost you've run into?