Paul, it's not just the cost it's also the storage life if a replenished developer isn't being used sufficiently. I haven't used Deep tanks since the mid 80's so went back to how I'd worked in my youth (at school) using 2.5 litre bottles.
It's worth noting that Kodaks capacity figures are quite conservative and in practice those numbers would usually be exceeded by quite a long way, without any problems, we were experienced enough to spot when the developer was close to collapse & needed replacing.
A major bonus of using a replenishment system is there are no chemicals to mix each time you process, so it's load spirals et and away, it's easy to keep or bring the chemistry to the right temperature, so it can cut the overall processing cycle as there's less preparation etc.
With replenished Xtol I was often processing 20-30 roll, leaving it stored a month or so before the next batches and my working solution lasting over a year with no problems, I would process a single film first to double check the developer was still OK. The only reason I've just stopped using Xtol this way is I'm rarely back in the UK, and it's easier to fly with concentrated developers, plus my Xtol doesn't get used/is stored too long.
If you process 3 or 4 rolls of film a week or 6-12 sheets of 5x4 or larger then using a replenishment system is definitely worth contemplating.
Ian