At one end of the spectrum, some people here claim to write this kind of software over a weekend. Some others claim they require a salary of $600,000, and still need nine additional colleagues to pull something like this off.
There is a lot of room in between, where cost estimates are more realistic.
Plenty of things can be prototyped over a weekend, but many will require months and even years to get production-ready, feature-complete, and useful, especially at scale.
This answer pretty much sums a lot of my experience. Of course when the guy somehow pulls this off in 2 weeks it is seen as an easy side project with proof that it is, haha
There is a lot of room in between, where cost estimates are more realistic.