The cost of being general

Cost per useful hour

Purchase price is the wrong comparison, because the two options fail differently. Use cost per useful hour: total cost of ownership over a year, divided by hours actually spent doing the job.

The denominator is where general-purpose machines lose. Charging, resetting, being repaired, waiting for a human to unstick them — none of those are useful hours, and all of them are more frequent when a machine is doing tasks it was not designed for. A conveyor does one thing at 99% uptime. Uptime is the whole argument.

Compare against a purpose-built alternative

Take the task from your economics decomposition — the one you already computed a payback for. Cost it two ways.

Purpose-built: the boring machine that does exactly this. A conveyor, a labelling machine, a scanner, a jig. Get a real price; most have public ones.

General-purpose: a mobile manipulator, at whatever price is currently quoted, amortised over three years, with an honest uptime estimate.

Then compute cost per useful hour for each.

The ratio between them

General-purpose cost per useful hour divided by purpose-built cost per useful hour. Above 1 means the boring machine wins today; below 1 means it does not.

Where the general case actually wins

Your ratio came out at 6× — the purpose-built machine is far cheaper per useful hour. Somebody argues humanoids are still the future.

What you would need to see

You now have three numbers about a category most people only have opinions about.