Off-the-shelf tools are built for the average business. Your margin lives in the ways you aren't average.
Every SaaS tool encodes an opinion about how your business should run. When that opinion matches yours, it's the cheapest software you'll ever buy. When it doesn't, the gap gets filled by people — copying data, maintaining shadow spreadsheets, and reconciling systems that were supposed to talk.
Custom doesn't mean expensive anymore. A focused build around one workflow is a matter of weeks, and it belongs to you afterwards — no per-seat pricing that grows as you hire.
The honest rule: buy the commodity, build the differentiator. Payroll is a commodity. The way you quote, dispatch, or document work usually isn't.