The point is that "app" means application. "What does an application cost?" - well, is it a CLI that prints the current time in the terminal? Or is it Microsoft Word?

You can get a fully functional well polished fart button app for both iOS and Android made for less than $4k, but I doubt you could make something like the Facebook app on even one platform for $400k.

That would defeat the whole point of the analogy which is to compare to something familiar. Nobody is familiar with buying "custom cars"

Custom houses is something that people are familiar with. It's an incredibly stressful and unpredictable undertaking for most people. But compared to custom software, its nothing. A custom house might cost 20% more than what was expected. It might take twice as long. Software might cost 2000% more than expected and the end result might be nothing like what the buyer though they were going to get.