FYI, based on my past experience, for avg urban customer the total acquisition cost for a cable ISP on DOCSIS 2/3.0 was about $750 -- small delta vs your new build cost.
The trouble is that Virgin Media have much cut out for them if they want to match the availability of VDSL in the UK. Therefore, I can imagine they will be targeting large towns on the commuter belt long before they consider more rural villages.