To answer the origininal question :
It is cheaper to restore an existing duplication than duplicate an existing single line , because the second category requires all new earthworks, bridges, culverts, viaducts etc .
E:G: To restore duplication between Kyneton and Bendigo one does it in stages
Stage - 1 Kyneton to Up end Taradale Loop, + Extend Castlemaine Loop to down end of Harcourt, restore double track section through Bendigo Yard.
Stage - 2 Down end Taradale Loop to Up end Castlemaine Loop + Down end Hardcourt Loop to Up end Kangaroo Flat .
The tunnels are not an issue PROOF Sprinters which are wider & longer than VLocity cars ran through both tunnels for years at 130kmh no problems, so narrower and shorter VL cars at 25m can run through both tunnels on restored parallel tracks quite happily at 130 kmh .
On an existing single track duplication depends on bridges viaducst earthworks etc . South Geelong to Waurn Ponds with two long bridges over the Barwon River floodplain is 17 kms around $ 300m , plus second platforms, signalling etc and two grade separations brings it up to $ 700m .
The additional cost of installing full size gauge convertible sleepers is around $ 1 per sleeper extra . The trouble is we dont have at this time a low profile gauge convertible sleeper, only full size gauge convertible sleepers . Austrak at Avalon could no doubt make a low profile dual gauge sleeper but no one at V/Line or Victrack has asked them to develop a design at this stage apparently .
One would only go for dual gauge sleepers in new work or any renewals on existing bg lines where there is a high likelihood of gauge conversion to sg in future like between Ballarat & Ararat and North of Wallan to Seymour , Shepparton and Tocumwal .