Looks like you enjoy shooters. I'm not totally sure on the exchange rate, but the first two Borderlands games are $20 right now.
Shooter/Roleplaying with Deus Ex: Human Revolution for $20 is a good call.
Also, for silly shoot/open world fun, Saints Row 3, IV and Gat out of Hell are all about $15 (Really don't need to play previous games in the series to appreciate them.)
On the RTS front, Warhammer 40K: Dawn of War isles than $15. It's a little old, but still very enjoyable.
You kind of ask a very subjective, wide-open question, so it's hard to just blindly pick something for you, but I hope this helps.