I think I've hammered out how repair works, Mk III engies just became a lot more important if you are going to run golems.
Repair is 1/4 the cost of building a ship for a ship with the Repair Rate 1 ability (such as the Repair Station). At higher Repair Rates it gets more effective.
The numbers are as follows:
Base Repair Time: 1/4 of listed Build Time
Base Repair Cost: 1/4 of listed Metal and Crystal cost to build.
Repair Cost per Second: Base Repair Cost/Base Repair Time
Take our example ship, 400 metal, 400 crystal and 400 seconds built costs, for a Mobile Repair Station with a Repair Rate of 1, it would take 100 seconds and 100 crystal/metal to heal from 0 health back to full health at a rate of 1 metal/crystal per second.
However, for ships with higher repair rates, only the Repair Time is affected. What this means is that Repair Cost per second never changes while the time to repair a ship goes down.
Take our example ship as being healed by a Engy Mk I which has a repair rate of 6.
Repair time is now 16.6 seconds ( 100/6 ) but Repair Cost per Second is still 1 metal/crystal per second so the Engy Mk I only consumes 16.6 metal/crystal to repair our ship over 16.6 seconds from 0 to full health.
This holds true for the Engy Mk III aswell at a repair rate of 12.
Repair time is now 8.3 seconds ( 100/12 ) but Repair Cost per Second is still 1 metal/crystal per second so the Engy Mk III only consumes 8.3 metal/crystal to repair our ship over 8.3 seconds from 0 to full health.
This means a Engy Mk III consumes half the resources to repair a ship that an Engy Mk I does.
And the Mobile Repair Station is awful, it consumes over 10 times the resources that an Engy Mk III does to repair a ship.
Wow, just wow. Maybe I'll actually unlock Mk II or III engineers now and not break my economy repairing my fleet. I always assumed the costs were the same, it just took longer with the Mk I engies, not that it was cheaper to repair with higher mark engies.