Of course, the more material and more space, the better gym you can build. But for most, even serious athletes, you don't need much more than a small room, a cage, bench, a few bars, and 700 pounds of discs. Although not comfortable, make detachable dumbbells if you don't want to take up a lot of space or spend a lot of money. Add some decent cable machine.
With an investment of 4000-6000 dollars you should be fine.
If you are a competitor and feel that you need to play with more variety, angles and machines, and train with giant sets, then you cannot have this at home unless you are Ronnie Coleman. So, occasionally join a gym.