$30 is what, £18?
Bastion is good. Very compelling. Currently £11.49
I recently got Godus on sale. Currently £14.99
It's a little bit lacking at the moment as it's in development, but I've found it addictive and look forward to the point that it is finished.
Kerbal Space Program is very good. I find it a little bit tricky, but I think I'm just a crap rocket scientist.
Currently £19.99, so possibly a bit over your price range.
Red Faction: Guerilla is a fun game. Freeroam and destructive. Little bit boring once you complete it fully, but still offers some good laughs and hours of entertainment. Currently £14.99.
Red Faction: Armageddon is also fun. It's not freeroam though, and is linear. But equally destructive. I found it felt slightly short, but still entertaining. Also currently £14.99.
Red Faction 1/2 are good games (although I only really remember RF1), and are entertaining. If you get RF1 then definitely at some point set up some Player Vs Bot matches. They're lots of fun. Good storyline and interesting gameplay.
Dated however. Currently £4.99 each.
If you were interested in Starbound, then give Terraria a good look too. Very similar, but bonus side is it is complete. Addictive game, particularly if playing with a friend. Depending on how well you can entertain yourself you might find it lasts forever as entertaining. I personally get a little bored come end-game.
Currently £5.99, but often comes down more in pretty much every regular sale.
