Making it safe for the properties above is a much easier task than safe for the passage of trains.
You could fill with foamed concrete for a few 100k, you might even bypass liability for mines drainage that way.
I'd be surprised if they managed to do that for 100K or even 300K, consider railway inflation

100K will barely build a house in 2025.
I'd agree it would be cheaper though.
I think this is a unique case of a situation where it would be hugely cheaper to close the tunnel and not that disadvantageous given the main passenger flows on the route and the simplification of operations running it as two branch lines would provide for (as I outlined above, hourly extensions of the Barrows to Millom with two hourly ish extensions based on Sellafield shifts to Whitehaven South/Corkickle* plus an hourly extension of the Newcastle-Carlisle to Whitehaven Central would be a very usable service pattern and very much in line with demand and would allow the Barrow side operation to be simplified to exclusively use Class 195 units** as well as offering stations a direct service to Manchester Airport which don't presently have one).
However I do still think the tunnel work will be done unless it proves totally impossible because it would be politically difficult to "abandon" the generally quite poor population of West Cumbria. I do however think it might take a while to do it. After all money is always found to patch up the Conwy Valley yet again (though I do think a major collapse in Ffestiniog tunnel would probably result in closure south of Betws on that route) and that's needed every few years, not once since it was built!
* That renaming might well make sense though Corkickle is such an odd, quirky name I'd love to see it on the big display at Manchester Piccadilly!

** In the event of insufficient Class 195s to do this, some could be released from the east Manchester locals, sending 15x there instead.