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).