It can often work out not only cheaper but also more flexible. For a start every depot has overheads both staff and building wise. They need to maintain the building, staff it with management, pay running costs etc. and often smaller depots won't be very efficient with the amount of work they would get from the drivers there as a diagram must obviously both start and finish at the same location. Having a bigger depot is cheaper with overheads and may require more passing work or taxis but also adds greater flexibility to the work.
Hitchin depot on FCC is quite a good example as almost all of their diagrams either start or finish with passing or taxis to/from letchworth, welwyn, Finsbury park, Hornsey, Hertford or Cambridge (they only have a handful where they both start and finish driving in and out of the station). There has been lots of talk over the past 30 years about closing hitchin and opening smaller depots at welwyn, letchworth and Hertford but a single depot is still cheaper and with pass times of 5mins to letchworth, 15mins to welwyn/Hertford plus 30mins to London or Cambridge and 45 mins to peterbourgh it makes it very flexible for both diagramed work and flexibility of spare drivers/disruption/cross depot cover for staff sickness etc. so it's not purely down to the headline cost but also about how flexible the depots locations are for getting around the rest of the network.