Not that it changes your point that much, but doesn’t MISO have more like a 20-30% Coal mix?[0]

It looks like natural gas is usually the biggest source of electricity.

[0] https://app.electricitymaps.com/zone/US-MIDW-MISO/72h/hourly...

If you look at the all years view on electricity maps (https://app.electricitymaps.com/zone/US-MIDW-MISO/all/yearly), 25% is share of generation from coal in MISO.

I'd be curious to test the GP's point. Since electricity maps doesn't have cost data for most US balancing authorities, you maybe could try figuring out power costs per balancing authority to end customers by using something like the https://www.eia.gov/electricity/data/eia861/ "Sales to Customers Customer Sited" data. Revenue over Megawatthours for Industrial service.

You can look on the real time map:

https://miso.singularity.energy/realtime

I am seeing 39.1% right now