Having had a similar thing done, the reason for this is that Apple will not replace a single key on the keyboard once a laptop is shipped in for repair.

In some stores, techs may try to re-seat the existing key, and in very rare cases have replacement keys to utilize in-store, but usually they end up wanting to ship out for a full replacement top case.

So you are not paying for a key, you are paying for a new Apple keyboard (which sadly, will inevitably develop the same key issues as the design is not fixed.)