I read a lot of reviews that they are different. But how, is omitted. The "X Rebirth: Home of Light Complete Edition" page in GoG only said that it is easier than X3. So I went to Wikipedia trying to find what is the difference. No explanation there either. Then I read a few threads in the forum here, none had the actual topic how do these games differ from each others.

What I have found so far, is that in X Rebirth you still command fleets but can only fly one ship and there is less commands you can give compared to X3, and so, it is easier to learn. Apparently X Rebirth is prettier and the User Interface is different. And you can walk inside space station, apparently in X3 you can't.

The price gap is HUGE, and X Rebirth is much newer game. So is X Rebirth an improved and simplified X3? How would you explain, how are the games different, in the big picture?

Ahaa, I didn't know that X4 is more like Rebirth than X3.

I had X-series games listed with my 4x interstellar war games like Stellaris, DIstant Worlds, Galactic Civilizations and such. Although I know it's simulation style. Apparently I should then move X Rebirth and X4 to the likes of Starfield, Star Citizen and Elite. To the interstellar life simulation.

The X games are indeed not really 4X games as such. Sadly I cannot compare anything to X Rebirth as I have never played it.

X Rebirth flopped is the reason for the price difference, that's pretty much it. It removed a lot of features in the name of addressing the horrible learning curve of the series and players were very unhappy with it, myself included. I don't think anyone disagrees that even X4 is clunky, but the series does something no other game series really does. Egosoft chucking that away because some people found it hard to navigate was definitely a questionable decision. I just checked my steam reviews and I wrote a wall of text for about how much I hated it was when I played it during a free weekend. Can't believe it was almost 8 years ago now. Game still has only 41% positive on Steam.

It might be worth a look now since Egosoft is pretty good about patching their games until they are of good quality, but I wouldn't put it in the high priority bucket by any means.