Yes I can well imagine the costs of installing the new system. Just wondering if the thousands of old signals might find a home, maybe in another country (the destination of some old locos I think?).
Very unlikely that the costs of recovering old signals in a usable state would be worthwhile. Unlike locos and rolling stock that can be easily moved around, signal posts and heads are not. You would need to get lifting equipment to site, to lower signal heads without damaging them.
As Bald Rick says, some of the components like trackside object controllers that can be easily recovered, might be recovered for spares. But to be used on a new signalling scheme, they would likely need to go back to the manufacturer for recertification, which is unlikely to be worthwhile.
It is also worth remembering that an overlay is what it says, it is something that is overlaid on an existing system. It doesn't work in parallel with it, it inherently relies on the existing system on which it has been overlaid - it won't work without it.
It may be possible to recover things like signals, but as Tim says, things like train detection, point motors, etc would need to stay. As would any lineside cabling that they use. Recovering the signals but leaving the other things would likely require a fairly major rework of the underlying system. As an example, in an overlay area, an ETCS movement authority is likely to prove that the exit signal is alight. Removing the exit signal will require the software and hardware amending to remove both the signal controls and proving.
To be honest, I doubt that recovery of lineside signals in a dual-signalled area is likely to be very cost-effective, until that area needs resignalling again.