The cost to process on a Jobo is not too dissimilar to the cost of processing yourself in conventional processing tanks! The Jobo merely helps regulate agitation and temperature, for better consistency.
I cannot think of a significant difference that results in the reason to compute an ROI period against the expense of purchase.
The chemical cost is a real factor. If you do roll film in a stainless steel tank, for example, you can get two rolls of 120 in a liter of chemistry. With a Jobo, you can do ten rolls in that same liter of chemistry. Theoretically you can get the same ten rolls out of the liter before it exhausts whichever way you process, but doing it in small lots over time, you'll probably not get ten rolls out of the same batch doing it in stainless, only 8. So that's a 20% savings on chemistry.
And while you'll use up the liter at the same chemical rate (see above for caveat), it will take you five times longer to develop those ten rolls in SS tanks. That's two-plus extra hours of time that could be spent printing, or shooting more film, rather than staring at a timer.
If you do b/w film, the savings is even more dramatic because your chemistry is always one-shot. So two rolls of b/w vs ten. You've cut your chemistry cost by 80%. So compound a time savings and a cost savings, and that's just comparing it against doing it yourself without the Jobo. Compare it to the cost of having that work done by a lab, and the justification is very easy.