Minion Damage: 15% of what?

The zombies are a monster. Like all monsters they have their own base damage, it's not based on your weapon. You can add base damage to them with support gems as well.
"DPS" shown on Raise Zombie is not a valid value, we just don't have a way to disable it for only certain skills yet.

It depends on the level of the gem, and changes whenever the monster balance changes. Eventually we might be able to hack that into the stats system so it can be displayed on the skill tooltip.