75% off. I consider this game the sumum bonum of tower-defense games.
It's a great game. If you're going to have one tower defense game, it's either this, or Orcs Must Die! 2.
Or Sang-Froid. (Although OMD2 probably exceeds it overall.)
For sci-fi TDgames I tend to rather like Savage Moon, but that doesn't seem available for PC.
This is by far the best tower defense game I've played. I'm tempted to buy this even though I have it on the sexybox already.