I don't know if this already been asked and as I fall into the "naysayers" on this thread that thankfully we have a "Speculative Forum" for on this website for, based on current 2025 costings, taking everything into consideration what is needed to bring this speculative project into fulfilment, what total figure would need to be spent.
The true answer is “nobody knows”, because every study (or ‘study’) done only looks at the Skipton - Colne section, and assumes that works off that section needed to accommodate the train service in a feasible timetable are done by “others”. (Others being taxpayers, of course). It is the same with almost every other old chestnut proposal.
But, I’ll have a go with an educated guess. My own thoughts and nothing remotely official.
Opening a new line from Colne to Skipton, given the need to build at least three big bridges, compensate a lot of property owners, have a completely new alignment around Earby, and complete a no doubt difficult consents process, I can’t see that costing less than £400m at outturn costs, and probably £500m+.
Add in some doubling of the existing Colne branch, Level Crossing upgrades to deal with the increased risk due to longer / busier trains, discontinuous electrification between Skipton and Preston, possible platform extensions Colne to Burnley (and possibly the Aire Valley) and that’s easily another couple of hundred million.
Then around £10m a year incremental cost of operation (all in - train lease, mileage, electricity, crew, maintenance, cleaning, infrastructure maintenance, operation, renewal, retailing costs, overheads).