A key cap falling off was never covered by the free Keyboard repair program. The free keyboard repair program only covered issues where the key press would not be recognized or where a key would repeat itself.
If the butterfly mechanism was not damaged, then it is possible to replace just the key cap. In theory Apple or an AASP should be able to do this, but because Apple only provides an entire keycap replacement kit the price may still be significant. Or it may be possible to purchase a key cap online from some other vendor, but this is risky since there are several different types of key caps, but only one that will be compatible with your laptop. Plus the color & texture of the key caps may not match your keyboard, plus the style of the lettering may be different as well.
If the battery condition becomes "Replace Soon", "Replace Now", "Service Battery", or "Service Recommended", then you can ask to have the battery replaced which will be a much cheaper repair (probably only around $200-$260 US). FYI, the battery is glued to the underside of the palmrest on the Top Case Assembly which also includes the Keyboard.
If a key cap replacement is not an option because the Butterfly mechanism is damaged and the battery condition does not allow the battery to be replaced yet, then you should check with an Apple Authorized Service Provider who performs repairs at their own shop. The $800 price for a Keyboard sounds like a price you pay when the laptop is mailed to an Apple repair depot. An AASP should be able to perform the repair for a little bit less (closer to $600 US). I know that is still way over priced for a keyboard, but unfortunately Apple consolidated multiple parts into a single repair part (Top Case Assembly) instead of allowing each component to be replaced individually like on 2012 and earlier models (or like their competitors). Apple believes their customers have money to throw around.
You can provide product feedback (Apple will not respond):
https://www.apple.com/feedback/
You can also contact Apple corporate to let them know about your issue and how you feel about the expensive repair just for a key cap (you may receive a response from Apple):
https://www.apple.com/contact/
You can also check your local consumer protection laws.
