When building your own computer, you can buy all the parts for a lot less than a pre-built computer, for sure.
However, it takes a fair amount of time to do the actual build. So, figure out what your time is worth....
Usually, when you include your time, the cost is actually higher than getting a good gaming computer.
The advantage of building your own is that you can vary the specifications to meet your exact needs, compared to having something close to your needs.