Elder Scrolls Online!
Why get one addictive game, when you can get like 5-10!
Oh. For $30 I would recommend one of the Civilization games. Specifically, if you're looking for a challenge, Civ3 is ridiculously difficult. I think it's like 5 bucks or something.
I've heard very good things about The Wolf Among us. 25 bucks.
If you're looking for an addictive game, you might be interested in an MMO. Tera is completely free and it is brilliantly beautiful. The story is pretty lame, though.
I would definitely recommend Rome: Total War. One of the best of its kind.
If you haven't played it, Fallout 3 is incredible, and it's only 10 bucks.
The Witcher 1 is only 10 bucks.
Hitman 2: Silent Assasin was pretty addictive for me: 7 bucks
Portal1 and 2, 5 bucks each
Vampire the Masquerade - Bloodlines 5 bucks
Half-life 2, 5 bucks
There are all kinds of great, addictive games for cheap if you're willing to go back a couple of years. And trust me, there are some amazing and cheap games out there. Try browsing through Steam. ^-^ Good luck to you in your adventures.