This is actually becoming somewhat commonplace. For example, in many cutting-edge cancer therapies, blood is drawn from the patient, processed in tissue-culture suites on site to extract the patient's immune cells and sensitize them to some marker expressed by their specific cancer cells, and then the modified immune cells are returned to the patient room and transfused back into their bodies. It's not cheap per se but it's something that most top-tier cancer centers can do, and to do the similar process of extracting stem cells, inducing them to transform into pancreatic islet cells, and transplanting those into the patient's pancreas isn't that big of a jump -- and it'd be cheaper than a lifetime of insulin in any case. It also points the way towards treating other kinds of organ failure without the risk of rejection, too.

I see you're describing a case-by-case basis, but I'm still failing to see how it's case-by-case. /s

That's not what case-by-case means. Wiktionary:

Separate and distinct from others of the same kind; treated individually.

Case-by-case implies that each treatment is different and is not generalisable; but the fact that they use a patient's own tissue does not make each individual treatment different. If you want to extend the logic, you might call vaccination a case-by-case treatment as well, since they use different needles for each person.