I'm no expert and I'm not pretending to have read up on the numbers (what's the point? It's basically impossible to find impartial analysis in this country, especially during an election campaign) but I will poke my head above the parapet enough to point out that the current government's own analysis apparently showed that it would be cheaper for broadband provision to be run by the state than to continue as it is now.
I just so can't be arsed reading people's takes on these things now. Sections of the media are likening nationalisation to communism now. I didn't do politics at school or university but even I know what communism is and what it isn't. Perhaps certain members of the media aren't so lucky.
As to the estimated cost, Labour say £20m, Conservatives say £80m, BT say £60m and you're saying £100m. I would bet then that the true figure is probably around £40m as then everyone has lied or exaggerated or underestimated sufficiently in their chosen direction. Can't trust any of it though, people just make numbers up willy-nilly these days to suit their agenda and barely anyone in this country with a loud enough voice and far enough reach is anywhere near impartial enough to provide real information.