theres not enough rich billionaires in the world to feed it.

If each harvesting of adrenochrome kills a person, and there are thousands, or tens of thousands of adrenochrome consumers, that's a lot of people required to feed that addiction. That's just one of the possible uses for humans. There's also humans as food, organ harvesting, scientific research, and sex, and that's just off the top of my head.

Think of them as a resource, like cows, or "cows with benefits" and you can perhaps imagine the possibilities.

a high risk , cost and for what?

That there exist multiple underground "cities" is not in question. See Denver Airport, NORAD, or even Manhattan for example. That there exist numerous "town" sized underground systems is also not in question. That there exist underground highways that connect these places is not in question. That they do in fact hide shit down there is also not in question.

What is questionable is, how many cities are there, how connected are they, what do they really do there, how deep do they really go, and are there some that even the military doesn't know about? Hell, there are underground cities that exist from before (thousands of years ago), though it would be more appropriate to call them "towns" I suppose, but they are very sophisticated (cisterns, plumbing, compartmentalization, etc.). They were almost certainly used for survival at some tumultuous period of our past. But who knows how many such "aforetime" places exist that the PTB know about that have been hidden from us?

There is NO RISK involved, because a) almost no one knows about these things, b) no one thinks about these things, c) no one can go there that isn't shown the way and allowed to get in, d) any "leaks" are controlled the same way that all other leaks are controlled, which is to say, trivially, when you own all the media.