I’m 2 weeks late to this conversation, but reading through the history I found a couple things I want to comment on.
While her response is obviously extreme, it didn’t feel unnatural to me. This was my interpretation:
One fact I think is very important to this whole interaction is that at this point, マイン’s fever is incredibly strong and she has to constantly exert effort to keep it inside (the metaphor of squeezing it into a box). She’s exhausted from this constant strain, and from the frequent painful flare ups that last multiple days, and there’s really no sign of it ever going away or even improving—in fact, it’s only gotten worse with time. On some level she’s probably resigned to the fact that it will kill her eventually.
It’s in this moment that ルッツ confronts her about the original マイン, something she briefly wondered about herself at some point in volume 1, but never came up again. The idea that she’s living in someone else’s body, a real person she even spoke to once, is a horrifying thought that she’s been avoiding for months now. On top of that, the fact that she’s basically been deceiving everyone around her has also been revealed. She can easily imagine that if ルッツ told anyone else, her current life would be as good as over anyway.
All this stress compounded into her response. On top of that, since she’s actively working to keep her fever from flaring up, she really could “just stop” and let it consume her at any time.
On a more lighthearted note, about the phrase
「お金は取れる時に、取れるところから、取れるだけ、取っておくものだ」
@Naphthalene’s translation is good, but I think the nuance is slightly different. ベンノ is reprimanding her for not taking enough money, and emphatically telling her the “correct” way to think about money.
Overall structure of the phrase:
お金は △△ものだ。= Money is a △△ kind of thing.
And then 4 clauses that all emphasize the verb 取る
取れる時に、= Whenever you can take it
取れるところから、= From wherever you can take it
取れるだけ、= As much as you can take
取っておく = Take (even if you don’t have a particular use for it yet). This one directly modifies もの, turning it into “something you (should) take”
So, with context phrases for emotion, I’d translate it something like
"マイン! Money is something that you should take any time you can, from anywhere you can, as much as you can! Got it?
I think everyone got the meaning already but I really like this phrase so I took the chance to pull it apart 
so I’m aiming to have that same schedule.
I think I’m back in the swing of things, though.
Looking forward to hearing your thoughts!