Doing it yourself is always cheaper than buying pre-made, with very rare exceptions where there might be a refurbished unit up for sale or some huge blowout sale. If you think you can handle putting one together yourself, I'd recommend doing so.
That said, it's gonna be tough right now to find parts for cheap. Covid has made prices go up because people being stuck at home means more demand to replace old PCs.
If you absolutely must and can't wait, for finding parts yourself PCPartPicker.com is your best bet. Not only do they help you find parts that are compatible with each other, but it shows the lowest price. (Despite the price finder, it doesn't hurt to shop around a bit yourself because I have found it's a little off at times.)
I would also recommend planning for the future and not buying low-tier parts or builds. If you buy the lowest tier that will get you by on med-low settings right now, you'll need to replace at least the graphics card again in a couple years max. If you get mid-high range, you could go for up to 10 years (but by the 7th or 8th year you'll probably find you can't play on max settings anymore).
My first PC was a pre-built Alienware. I didn't know what I was doing, I just wanted a pretty PC to start gaming on. I had to replace the CPU (and with it the mobo and RAM) last year when the CPU finally died. I had it for 10 years before that and was still able to play most things on high by the time it died (sometimes only getting 30fps, but I'm not a stickler there). When I bought it, it was considered high end, but not the top tier you could get. In that time I upgraded from 2 crossfired ATI somethingorothers to a GTX 660 and then a 970, added 8GB more RAM, and... a lot of hard drives. I still have the same case (pretty lights, yay), drives, and 970 in my current rig. It still handles most things with ease. For Cyberpunk, I have it set to a sort of medium to high and get a solid 40fps+ unless in the first sort of central area, then I drop to 25-30fps and crash a ton (but the crashes are common for lots of folks, so it's the game more that it is me).
tl;dr - Build yourself later rather than now, pcpartpicker.com, don't buy low end unless you're ok with replacing things again in a couple years, Cyberpunk is broken.