Ever heard of Overgrowth? You may know of Wolfire Games, the developers of Overgrowth and organizers of the Humble Indie Bundle. They currently charge $30 for the alpha of their game which is little more than a (rather impressive but content-lacking) tech demo. Starsector has given me hundreds of hours of playtime for a third of the price, and I've owned the game since when only the missions were available. Since I've purchased the game, there have been many great additions, like a campaign mode, more ships, ship systems, the ability to outfit ships, character leveling, and dozens of improvements in AI, GUI, and overall design, with so much more planned.
Buy it. Buy it now.