"I have the 2020 13" M1 MBP. Currently the battery is 94% so not the biggest issue but how low do you let it go before you replace it? What's the cost and process?"
Egads !!
There's nothing wrong with a battery that's at 94% of original capacity.
It's a process called "aging". Happens to all things.
When the capacity drops below 80%, then you can avail yourself of Apple's battery replacement program, which costs $249. You get a new Apple-labeled battery, installation, and a short warranty.
"Apple labeled" can make a big difference. Seems like 3rd-party batteries are a crapshoot. As Forrest Gump said, like a box of chocolates: "you never know what you're going to get" ...