When custom software is cheaper than software

Usually it is not. The exception is specific enough to describe.

Off-the-shelf software is nearly always the right answer. Somebody has already solved the problem, tested it against thousands of businesses, and spread the cost. Building your own version of it is an expensive way to get something worse.

The exception is when the thing you do differently from everyone else is the thing the software will not bend to. Then you are paying twice: once for the licence, and again in the hours your staff spend working around it, every week, for ever.

That second cost is invisible because it never appears on an invoice. It shows up as spreadsheets beside the system, as data retyped from one screen into another, as a person whose job is partly to be the integration.

Count those hours honestly before commissioning anything. If the workaround costs half a day a week, it costs about twenty-five days a year, and that is the number to compare against building.

And when the answer is to build, build the smallest thing that removes the workaround. Not a platform. The one process that is costing you, done properly, connected to what you already have.