This is just a touch over $800 and I did not select a case, but it has some premium parts. You could go for a lesser monitor, but 144hrz monitors are very nice. You could also swap the Ryzen 3 for a Pentium G4560 and save some cash, but I think it is worth the extra $60 is worth it for the long run.
PCPartPicker part list:
https://pcpartpicker.com/list/svzb9W
Price breakdown by merchant: https://pcpartpicker.com/list/svzb9W/by_merchant/
CPU: AMD - Ryzen 3 1300X 3.5GHz Quad-Core Processor ($129.00 @ Amazon)
Motherboard: Gigabyte - GA-AB350-Gaming ATX AM4 Motherboard ($86.89 @ OutletPC)
Memory: Corsair - Vengeance LPX 16GB (2 x 8GB) DDR4-3200 Memory ($139.99 @ Amazon)
Storage: Western Digital - Blue 250GB 2.5" Solid State Drive ($89.88 @ OutletPC)
Video Card: Asus - GeForce GTX 1050 Ti 4GB Phoenix Video Card ($144.98 @ Newegg)
Power Supply: EVGA - BQ 600W 80+ Bronze Certified Semi-Modular ATX Power Supply ($55.89 @ OutletPC)
Monitor: Acer - GN246HL 24.0" 1920x1080 144Hz Monitor ($188.99 @ B&H)
Total: $835.62
Prices include shipping, taxes, and discounts when available
Generated by PCPartPicker 2017-07-29 23:22 EDT-0400