I don't *think* broken golems have any special modifiers, per se. If the repair mechanic remains as it was since the 4.0 Unity release, then it's meant to be 1/4 cost and 1/2 time scaled with engineers. However, in my games, I've usually felt that repair costs for those golems were somewhat lower than that. Perhaps all repair is at 10% of build cost.
Search in the wiki release notes hasn't shown anything beyond 4.00. Also, the completed Regen Golem has another 90% reduction in repair costs.
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I think I've got it. The missing information is that the repair rate also decreases the total repair cost of the item you are repairing. I'll copy the procedure here for replication:
0) Use cheats, sand-box AI is best.
1) Starting planet should hopefully be 6/6, or 5/7~7/5 in m+c distribution. Spawn 30 ZPGs (3xenergize) to remove the home command station contribution, the rest is insignificant.
2) Unlock and build full caps of every engineer, unlock the MRS for Repair rate 1 comparison. Switch on only one group at a time.
3) Configure 999,999 m+c
4) Save now is best, no-need to keep typing m+c cheats.
The spot tests of the various golems and repairer combinations are as follows:
39 Eng I (x6 rate) + cursed => ~300K m+c over ~ 30 sec.
26 Eng II (x9 rate) + cursed => ~200K m+c over ~ 35 sec.
15 Eng III (x12 rate) + Cursed => ~150K m+cover ~ 40 sec.
9 MRS (x1 rate) + Cursed -> full 1M usage at ~65%, after several minutes.
26 Eng II (x9 rate) + Botnet => ~800K m+c over ~ 35 sec.
26 Eng II (x9 rate) + 15 Eng III (x12 rate) + cursed => ~175K, not timed (faster).
We will now carry out "magnitude-maths". Wrecks begin at 25% health, and repairs are 25% build costs:
Cursed Golems are 10M -> Nominal repair price is 1.875 M. This matches MRS usage of 40% ~ 1M. Also equal to (Eng repair rate)*(final cost). Botnets are 40M which is conveniently 4x cursed golem costs. Combinations of different engineers seem to work as a function of (repair rate)*(Eng numbers), so that cost contributions are roughly evenly split between full caps of II and III.
As for repair times. Build time is 720 minutes. Repair times are half that.
Repair time (min) with full Eng-III = 360 / 15 / 12 -> 2 minute for a full repair. Doesn't match with ~40 seconds, but it seems to be another constant factor - perhaps repair times are about 1/6 of build time?
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Conclusion: Repaire rate is both rate increase and efficiency increase, i.e. the old threads are correct. You definitely don't want MRSs to repair your golems!