I would expect to spend at least $1000 if you send it to Leica to replace the element. Leica cannot "repair" the scratch any other way. There are very expensive services that polish and re-coat lenses, but they're meant for cinema optics that are already worth around $50k, so the cost is relative there.
If that's worth it to you, then sure, but I don't think you'll make it back in resale value. Like everyone else has said, it will not affect image quality, especially being that small and on the front.