Cumulative cost of a monthly retainer against building and maintaining the same thing, and the month the two lines cross. Written to be fair to the retainer, and it will tell you when keeping yours is the right call.
Building costs £47,600 less over this horizon, and you own the result at the end of it.
Building costs less from month 14 onward.
Every month after that is £2,100 that stays in the business, and you hold the asset rather than renting access to it. You have already paid £35,000 on the retainer, which is spent either way and should not affect the decision ahead.
Those are real and arithmetic cannot price them. If what you need is continuous attention rather than a thing that exists, a retainer is the correct purchase and the crossover month is beside the point.
It would be, if the model were rigged. It is not. When upkeep meets or exceeds the retainer the tool says building never pays back, and when the crossover falls beyond your planning horizon it says the retainer is the sensible choice. Both outcomes are common with small retainers.
Ongoing attention, flexibility to move priorities month to month, no capital outlay, and delivery risk sitting with the supplier rather than you. All real, none of them priceable by arithmetic, and all listed on the page rather than left out.
Because nothing runs unattended forever. Hosting, dependency updates, the occasional change. A build quoted with zero ongoing cost is being undersold, and the true figure will surface later as an unbudgeted surprise.
No. That money is spent either way, and the only question that matters is what happens from here. It is shown because people want to see it, not because it belongs in the maths.
However far the business actually plans. Three years is a reasonable default for a website or an internal system. If you genuinely do not know whether the company will want this in eighteen months, that argues for renting rather than owning.