Well, this seems like the most appropriate thread for this.
I got twenty bucks left in my account, and I'm torn as to get Rust, space engineers, or something else.
To clarify: I want a simulation game, preferably where you can build, that can give me lots of hours playtime.
Any advice?
You can also browse GoG. You're looking for anything made by Impressions Games or Tilted Mill, since they're basically the same studio and they've been the biggest names in city sims that aren't SimCity for pretty much ever. Caesar 3 is very aged and has some mechanics that are difficult to deal with that later games dealt with. Pharaoh/Cleopatra doesn't matter that it has aged, it's very good and very solid. Zeus/Poseidon came after that and it's a further polished game with lots more content and a different flavor. Emperor: Rise of the Middle Kingdom was the pinnacle to this flavor of city sim (that being an isometric grid with 2d pre-rendered graphics, and the graphics look damn good by the way).
After that there was a Rome game I didn't really get into so couldn't tell you much about, but there was also Children of the Nile and its Alexandria expansion. This (and that Rome game) departed from 2d pre-rendered to full 3D, including things like day/night cycles, flood cycles, etc. They also move further away from the importance of mythology directly affecting the lives of the people and more into superstition. Children of the Nile is a bit dated, but because it's a 3D game it shows its age much faster than 2D pre-rendered games. And as much as I just poured time and time into P/C, Z/P and E:RotMK, I never could get terribly far into Children of the Nile. It's definitely a solid city sim, so don't take that as me meaning that it's a bad game or anything. It's more like being used to mom's chili all your life and then having a friend's mom's chili. Sure, it may be good, but it's really different.
I would whole-heartedly recommend Emperor, Zeus/Poseidon and Pharaoh/Cleopatra, in that order. For $20, you may be able to pick up both Emperor and Children of the Nile.
Otherwise, for city sims you have SimCity, Cities XL or a transport sim that's not really a city sim.
Ninjaedit: Also, the stand-alone DayZ is a very bad idea. Attempts to make a stand-alone were all well-received until it was apparent that the devs just wanted to gut every living being of all of their money and leave bleeding carcasses everywhere. I'm not sure if it was "DayZ" specifically or not, but one of the games and the devs got fucktons of flak for very poor business ethics towards consumers. I wouldn't go anywhere near a DayZ-type game without seeing lots of reviews.