When do you decide to build vs. buy for internal tooling?

We keep hitting this question: should we build an internal tool or integrate an existing SaaS? Recent example — incident postmortem workflow. Options: (1) build on top of our existing ticketing system (cheaper, more control), (2) buy a dedicated incident management tool (faster, but another vendor lock-in), (3) hack together a Notion template (fastest, least scalable). The team is split. What framework or criteria do you use to make this decision? Looking for something more structured than 'it depends'.