Paying for prebuilts is generally more expensive, however PCpartspicker doesn't really give the most accurate prices of all the parts (because it just assumes you're buying the cheapest option available even if that option is from a sketchy seller you wouldn't want to buy from or other things like that). Still...yeeeaaaah.

I had to really budget-shop when buying parts to build my PC...just keep an eye out for sales/fluctuating prices, you may be able to get a good deal on things.

Edit: Oh also, I had that case before. One thing with it is the back panels are gone forever once they're popped out, so if you keep that case, you might have extra open back panels, which let in dust...so extra dust filters might be needed. (The cable management in that case is also practically nonexistent.)