Simple but enjoyable
Saying you have a game which involves keeping back hordes of zombies is no longer a particularly novel concept. Having a game which includes the undead, and is done on a low budget is bound to be a challenge, with the now overwhelming crowd of zombie based games. Survivor Squad does a good job of rolling together some novel concepts with a top down shooter mechanic, that somehow captures the bleakness of a post apocalyptic world.
Each level has a number of buildings, with rooms and of course zombies. Using a variety of weapons, which you can find or craft, your small group of survivors must clear the stage / complete other objectives. There are different any types and small puzzles and traps to mix things up. But there is another drive for each level too - to find the best materials to salvage. This is because each time you finish a level, you are taken to a meta game which requires you to feed your survivors, kit them out and generally ensure you are ready to fight the next wave. You can travel between locations (using up precious gas of course) and even set up new bases.
It's not an easy game, or the most complex, but at the time of writing, it is available at a staggering £5.99 on Steam, and was in the holiday sale for a whole £0.83!