Using your calculations, that suggests that closing the Rochdale and HNC would cost CART something like £65M -- plus the other direct costs such as draining/making safe, so let's say £70M -- this could easily be more (£75M? £80M?) depending on closure costs.
CART annual spending on the canals and rivers is about £200M per year, which covers everything including rivers and reservoirs. For the sake of argument, let's assume they send £140M on canals (could be less?), so the closure costs would be at least half of the entire canal maintenance budget for a year.
How much money per year would closure save? The Rochdale and HNC total 53 miles long, which is about 3% of the canal network. If costs were equal per mile over the network closure might save £4M per year. Now these canals do have a lot more locks than most (about 120, about 6% of the network total IIRC) but then they're also used far less often than many others, which should reduce wear and tear.
Some of the numbers are (educated) guesswork, but it looks like it would take between 10 and 20 years for CART to make up the closure costs from annual savings -- and in the meantime they'd take a massive financial hit, which they'd presumably have to borrow money to finance. At 5% interest rates this would cost them about £3M a year at the start, so maybe £15M over the 10-year payback period -- which adds about 20% to closure costs, which means we're now up to 12-25 years break-even period, maybe even longer.
I can't believe that anyone with any understanding of business would think that this is a good idea... 😞