1) In 'Into the Storm,' the Soverign Venture is represented as costing 31 SP, far less than the Tyrant class cruiser it is based on. The Erata v1.3 corrected this, but to 70 SP. I've tried, but can't build it for less than eighty SP. Is my math wrong?
2) Is it possible to salvage and reuse ships or components captured from Chaos? Can Chaos warships be clensed, resanctified, and put back into honorable, holy service? Or is destruction the only option?
1) I can't really explain that. I do believe a few of the other ship packages have the same problem, but I always foresaw it as them getting discounts on a ship that is pre-made for them. 10sp is a big difference though.
2) It's a Houserule. There are no explicit rules, fluff or otherwise, stating you can't do this. Actually, come to think on it, I do believe a Space Marine Cruiser or Battleship was rescued this way. The ship was under the influence of Chaos, Imperial Space Marines boarded it, took it over, and converted it into an Imperium vessel for their Chapter. Now of course, this is Space Marines we are talking about, not a RT. I stipulated to my group when they captured a Chaos Raider, that it would take several years to a decade to consecrate it, and that is if they could convince an Ecclesiarchy group that it is worth it, who in turn would have to convince the Imperial Navy to redact the classification of the vessel from "Chaos tainted, shoot on sight" to something else. So, IMHO, it can be done, but only if the ship is worth it. Of course, having it "cleansed" by questionable Ecclesiarchy factions who don't report it to the proper authorities, and then using the vessel for pirate duties is always an option too. Though I would think it would gain several bad past Histories, with no good traits. Like Haunted with -10 (or -15) Morale, no bonus to detection, and possible a -5 to your defence against boarding/hit run actions, and then Turbulent Past with Imperial Navy, Inquisition, and other Ecclesiarchy factions, all with a -30 Social test and shoot on sight. Just an idea.
Your welcome, though I didn't help much. Best of luck to you and your players.