The decision is usually framed as cost, which is the least useful lens. The better question is how much of your competitive advantage lives in a process that no packaged product models correctly.
If your workflow is standard — invoicing, generic pipelines, basic stock — packaged software will beat custom on time and price, and you should use it.
If your workflow is the reason customers choose you, forcing it into a generic system usually means paying twice: once in licences, and again in the manual work people do to bridge the gaps.
A pragmatic middle path is common: keep packaged tools where they fit, build custom where the process is distinctive, and invest in the integration layer that keeps both honest.