The 12700k competes with the 5900x. The 5800x competes with the 12600k. The 12700k and 5900x need the same class cooler. The 5800x and 12600k need the same class cooler.
The total platform costs are a wash. Most of the cheap Z690 boards offer better connection options than the cheap x570 and b550 boards. Unless you are upgrading to a Zen 3 CPU, or have a very specific use-case, it doesn't make sense to build a new system with Zen 3 right now. If there are additional price cuts then we can debate it then. Right now, Zen 3 needs to come down further in price to remain competitive in new builds.
PCPartPicker Part List:
https://pcpartpicker.com/list/7MCk68
CPU: AMD Ryzen 9 5900X 3.7 GHz 12-Core Processor ($494.00 @ Newegg)
Motherboard: MSI X570-A PRO ATX AM4 Motherboard ($159.99 @ Amazon)
Total: $653.99
Prices include shipping, taxes, and discounts when available
Generated by PCPartPicker 2021-11-28 09:41 EST-0500
PCPartPicker Part List:
https://pcpartpicker.com/list/67Tcqp
CPU: AMD Ryzen 7 5800X 3.8 GHz 8-Core Processor ($340.99 @ Amazon)
Motherboard: MSI X570-A PRO ATX AM4 Motherboard ($159.99 @ Amazon)
Total: $500.98
Prices include shipping, taxes, and discounts when available
Generated by PCPartPicker 2021-11-28 09:41 EST-0500
PCPartPicker Part List:
https://pcpartpicker.com/list/dNj8LP
CPU: Intel Core i7-12700K 3.6 GHz 12-Core Processor ($414.89 @ B&H)
Motherboard: MSI PRO Z690-A DDR4 ATX LGA1700 Motherboard ($209.99 @ Newegg)
Total: $624.88
Prices include shipping, taxes, and discounts when available
Generated by PCPartPicker 2021-11-28 09:42 EST-0500
PCPartPicker Part List:
https://pcpartpicker.com/list/8YvzBc
CPU: Intel Core i5-12600K 3.7 GHz 10-Core Processor ($299.99 @ B&H)
Motherboard: MSI PRO Z690-A DDR4 ATX LGA1700 Motherboard ($209.99 @ Newegg)
Total: $509.98
Prices include shipping, taxes, and discounts when available
Generated by PCPartPicker 2021-11-28 09:43 EST-0500