Amazon has Micro Center Motherboard CPU Combo GIGABYTE Z390 Gaming X Motherboard ATX 2xM.2 Realtek ALC892 Intel LAN Bundle with Intel Core i9-9900K Desktop Processor on sale for $458.84. Shipping is free.
Other "cheap" CPU motherboard bundles available too:
CPU/i5 9600K + Motherboard/B365 HD3 $274.99
CPU/i7 9700K + Motherboard/Z390 Aorus Pro Wifi $412.29
CPU/i7 9700K + Motherboard/Z390 Gaming X $372.39
CPU/i9 9900K + Motherboard/Z390 Aorus Pro Wifi $519.99
CPU/i9 9900K + Motherboard/Z390 Gaming X $458.84