The US produces most of their own nitrogen, but the same is not true of potash.

The US does have potash mines for example around Carlsbad New Mexico. But these cover only a percentage of domestic need. Perhaps they could be scaled up not sure.

Also famously near Moab, Utah.

Isn't the stuff west of Green River, Wyoming also potash?

The US provides a lot of its own supply there.

Nitrogen is pulled out of the air which is free but the process requires hydrogen which is acquired from disassembled methane, the price of which is a significant contributor.

US shale oil notably has a lot of natural gas - methane.

the methane isnt the issue, building and operating the processing plants is

Hard to find exact data but 60-80% of the cost to manufacture ammonia comes from the cost of natural gas. Feel free to look a price charts for both to see the correlation.

Long term yes, however the building in such a plant is very expensive in the short term. As such, no one is going to build a plant unless they actually think it will have reasonably high utilization.