30%percent salvage isnt too bad, and I believe you can upgrade higher with vasari.
Yeah, Vasari can upgrade it to 50% salvage returned. BUT... scuttling the Starbase itself DOESN'T return any money! You only receive salvage money for the upgrades.
So that large original investment in the SB's is lost forever, but it is possible to salvage some of the value from upgrades.
I don't know if this is a Bug? It seems intentional. Although harsh.
Tip: If you research the Vasari tier 1 salvage tech upgrade before you scuttle your cap ship factory, the 20% additional salvage value will essentially pay for itself (the tech upgrade).