Probably around $1000–1100 ($550 for an RX 6800, $200 for a Ryzen 5 3600, and $300–350 for other random crap - motherboard, case, power supply, RAM, storage, etc.). It will actually be a bit faster, though, since the RX 6800 has 16 TFLOPS theoretical max throughput, while the Xbox Series X has 12.