Quite cheap but to compare, you also need to add the cost of the touchscreen and the board to drive all this, and your result would probably not pass EMC requirements (for a hobby project, you wouldn't care).
Still cheaper to source these parts yourself, you are right, but the official screen isn't meant to be the cheapest, it's meant to be cheap enough to be affordable, to workout-of-the-box, to be reliable, of good quality and have some guaranteed availability.
You can work with other screens and build and write your own interface if that's part of the pleasure you get from hacking on these devices, but for people who have other goals, being able to get an affordable screen that just works allows them to spend their time on other parts of their project.
They provide a model with a touchscreen for an extra $7. However, you can't work with other screens and build and write your own interface because on the Pi, all the display modesetting is handled by the closed-source binary blob running on the undocumented parts of the GPU, and the RPi Foundation won't enable DSI connector support in it for anything other than the official screen they sell. It's a fairly common hobbyist thing to do on more open boards, it's just not possible on the Pi.
Still cheaper to source these parts yourself, you are right, but the official screen isn't meant to be the cheapest, it's meant to be cheap enough to be affordable, to workout-of-the-box, to be reliable, of good quality and have some guaranteed availability.
You can work with other screens and build and write your own interface if that's part of the pleasure you get from hacking on these devices, but for people who have other goals, being able to get an affordable screen that just works allows them to spend their time on other parts of their project.
It's good to have choice.