I agree with many of the others here. It depends on what you want to do with it. If you are just doing some web browsing and steemit posting, you can probably get by with buying a PC from the store. My last laptop I purchased from the store for a fairly cheap price. Then I spent another $75 and added an SSD. That sped it up significantly.
If you are more into gaming or high performance you might want to look at building one. Then you can replace the components. Just remember the higher you want to go the more it is going to cost. I have a custom machine I just built for my office. I was able to put the graphics card I had in and then after a short amount of time I had enough to upgrade the graphics card to something better.
The big expense is going to be your video card and your processor if you decide to build one yourself. The case, power supply, motherboard, and hard drive don't have to be that expensive. Cooling for the processor can be purchased relatively cheap. RAM is middle of the road compared to the other components. If you love tinkering around and want to tweak the performance, I say build one.