I do have a gigantic external GPU which almost doubles the size of the entire system, and because of that I also need a decently big power supply. But the entire system is literally just incredictlmotherboard + attached GPU + power supply.
Being able to fit it all into a NZXT H210 (still relatively big compared many other SFF cases) is great, especially for transporting it between college and home.
It was about 50% cheaper than buying an equivalently specced laptop. And any laptop with this performance would need to be really big and heavy to accommodate the cooling requirements (45 watt CPU, 220 watt GPU).
AMD 5700xt GPU, $450 for the better versions
512 GB Corsair Force MP600 (5gbit/s NVMe PCIe 4.0 SSD), around $115 iirc
32 GB DDR4-3200 RAM for around $90 iirc
ASUS mini-itx X570 motherboard, around $200-250 - X570 is the highest tier of AMD chipsets but is needed for PCIe 4.0. Could save $100 here with a lower SSD. This one also has the latest Intel WiFi 6 chipset.
$100 for power supply, $100 for case, $30 for quieter fans.
Including tax and everything the total came out to around $1600. After deciding the specs I spent some time looking for comparable laptops and they were all much more expensive. I didn't do anything special to save. You could get this exact same build for a bit cheaper today I think; I did this in November. I just purchased a second one of the same GPU this week and it was $20 cheaper. The other components have probably also had some drops in price.
But I don't play many games that need GPUs and I don't mind low settings and terrible framerates.