but the developer is just one of 6 chemicals, so the overall savings for each roll must be very low?
Well, I thought you were only asking about the developer. BTW, you mention 6 chemicals - C-41 only uses 4 (not counting washes). I see you have a newer post about E-6, perhaps that's what you're asking about? I don't think I can help much with E-6, having never used it.

Regarding C-41, there are basically only 4 chemicals. Going off the top of my head, I'd say, for a single use operation, chemical cost would run, from high to low: bleach, developer, fixer, and stabilizer (very cheap). Bleach's high cost is in the actual bleach component.

If you were to run a replenished system, costs, from high to low, would be developer, bleach, fix, and stabilizer. Bleach has become much cheaper because you are no longer throwing away so much of the bleaching agent. Overall, the cost when replenishing everything is probably(?) more than 5 times cheaper, as an off-hand guess.

By the way, I'm not even sure that you CAN use replenished developer in a Jobo rotary machine; oxidation might kill it in short order. It looks like other posters, who DO use Jobos, don't replenish, so they probably could answer this. My own experience is mostly high volume - more than mini-lab machines could do; in these situations, single-use would not even be considered. But when your processing volume is low, and the chemical cost becomes trivial compared to your time, then single-use perhaps is sensible.