What is it going in? a small trunk, behind the seat of regular cab pickup, a land yachet that room to spare.
What are you looking for sound wise? A little more depth your music, being heard down the block, or competition.
What kind of box are you wanting? Those aren't cheap.
How much power are you wanting to push? If you go for lot then you are looking at wiring the "big 3".
Is the rest of your stereo stock? If so you will have to budget in a line driver.
You will hear alot of SSA, Sundown, and Obsidian recommended. Depending one space constrainsand what kind of output your looking for, l like a 12in sub. A used speaker in good shape is definitely a good way to save money. There are some really good amps in the classifieds too. If you get a sub and amp for 100ish ea.,You still have 3 for a good box, wiring kit, and line driver. If you can read a tape measure and work a saw, you can get a plan for $30 or so and save alot in labor. A sealed box is very simple to make.