How much does it cost to replace the battery for a 2020 MacBook Pro 13 inch M1 with 80% capacity?

It may depend on your location, but Apple has a service cost estimator on this Support page:

Mac Repair and Service - Apple Support

For your model it shows US$249 before tax and any shipping.

Anything other than a genuine Apple battery service will disappoint and fail to give you the runtime you need. Non-Apple batteries have a dismal track record for both runtime and service life regardless of who sold them.

Batteries in Retina Macbook Pros are not user-serviceable. The solvents needed to remove the old battery are hazardous and cannot be used in a retail setting. When you send it to Apple for service, it goes to a centralized refurbishment facility where they usually install a complete upper deck assembly including keyboard, trackpad and new battery.

if under 80 percent innate capacity, it can be replaced in the US for about US$250.

But why don't you just connect the power adapter when AC power is available? Are you working from the beach?

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That computer is a battery-CAPABLE device. It is not optimized as a battery-operated device. (It is NOT an iPhone.)

Your computer performs best when connected to AC power, such as the power adapter. It can use the full output of the Power Adapter AND when doing especially challenging work will also freely "borrow" power from the battery. In some cases, even with the power adapter connected, the charged state may decline during very stressful work.

When used only on battery, your computer has no extra cushion of power, and may perform more slowly. However, for ordinary non-stressful tasks this may not be objectionable (possibly not even noticeable.)

In general, you should ALWAYS connect a power source when it is possible to do so, and only run on batteries (which could be somewhat slower) when no power sources are at hand. Your Mac maintains its battery charge levels under program control, and will NEVER over-charge. 

Here is one more bit of trivia that can make power adapter use easier.

Apple sells a six-foot AC "extension cord" for the power adapter, which can make plugging in FAR easier: