What evidence is there that the Saturday strikes are costing the government much money? Northern trains are among the highest subsidised services in the country. If a reduced timetable is being run and little or no late services, then drivers are not being rostered, station staffing may be saved, BTP costs may be being reduced and so on. NR costs are mainly fixed, but most of the lines have other services running on them. Doubtless these services are filled rather 'cosily' but may be alternative solutions for many pax.
Perhaps there are bus services that were receiving public subsidy previouly that are now profitable.
I just wonder what the true overall financial effect is; a good task for an investigative reporter who doesn't just write his articles from Twitter?
There would be no savings to speak of from wages of drivers, station staff and others. They will all still be being paid as normal - even if they are sat around doing nothing, or are sent home part way through their shift.
The savings will come frkm not having to pay the guards for a day, from lower fuel or electricity bills and lower mileage on the rolling stock - all pretty marginal. There's debate about the impact on track access charges.
This losses will mainly be the extra hours paid to the contingency conductors and ticket examiners, and the reported bonuses they receive, and the reported overnight accommodation they need, plus any taxi bills. There will also be lost revenue and higher than usual compensation payments to people who booked tickets before the strike wad announced, and to season ticket holders who get a daily rate back.
Taking one day in isolation, I doubt that there is any money to be saved. If there is, it's likely to be pretty marginal. In the long-term, once they inconvenience, repuataional damage and direct repeat business (e.g. People who've refunded their annual because they've got sick of the disruption) is taken into account then it's clear the losses are very significant. This year's Estimates of Station Usage show a very significant pattern of decline at lots of suburban stations served by Northern.