If it's just a keycap (and maybe even the butterfly mechanism) that shouldn't be too bad.

https://www.youtube.com/watch?v=4KuVvb9DTaU (from about 5:20) talks a bit about what's involved in an actual keyboard replacement, if you ignore some of the histrionics.

I think from what I've heard that for actual replacements, Apple will swap out the whole 'top case' subassembly, which could be a 4-500$ part if it's not covered under warranty.