Port Wander is probably salvageable. It was implied to be intact enough that some Imperial survivors were still holding out in sections which had not lost atmosphere. Even if a lot of areas need a comprehensive rebuild, or essentially new sections to be grafted onto them, doing that piecemeal is probably a lot cheaper than finding another Ramilies Class Starfort, and having it brought in on warp-tugs. Also spreads the cost out over a longer period of time, which is probably preferable to Battlefleet Calixis.
Actually, that's potentially a thing House Scourge could do - help fund part of the rebuilding of Port Wander, in exchange for some shares of the trade going through it, and giving us some permanent presence and branding there.
After all, nothing says "you've made it" as a Rogue Trader dynasty like having your name and heraldry in embossed gold right above the doorway to the Koronus Expanse.