The average may be quite a bit lower for PvP, I have not tested as I have the time to complete with PvP off and it earns me more construction experience. Added the PvP value to the post.
It is currently possible to gain metal shards by building after capping. I did this on a number of days in the second week and it does work out to give a fair number - if you AFK and watch something at the same time you can get ~ 50 - 100 an hour for minimum effort. Again, this was with PvP off.