How on earth could Helen afford to buy the potion years ago? She was only successful after she drank it. Before that, she got evicted because she wouldn't pay rent.
Explained in the film: Lisle says the price is different for each person, and Helen later says it cost her "everything [she] had, which wasn't much".
As a Wild Mass Guessing, Lisle probably also uses the money partly as an excuse. She makes it costly to the buyers because people expect something like that to be fantastically expensive, but the thing she's really interested in is the people themselves... remember, she's assembled quite an eclectic cast of some of the biggest stars and celebrities in history, presumably far more than we even see. She may have given the potion to Helen for effectively nothing just because she knew it would eventually involve Madeline as well, and she was eager to see how things would play out between them, like her own little personally-engineered real life Soap Opera.
Helen actually says it cost her "everything [she] had, and that was cheap." That makes it sound like she was happy to pay the price, despite it being expensive, rather than sounding like she didn't pay much because she didn't have much, as suggested above.
How did Helen even find Lisle and the potion anyway?
Possibly through a weight-loss clinic or something similar, the way Madeline finds her through a spa. The potion doesnt make you lose weight, it just rejuvenates your age, so Helen would have had to lose weight on her own. Alternatively, Lisle had one of her contacts in the asylum where Helen was kept, and he gave her Lisles info.
What exactly is holding up the top half of Hel's body when she has the hole blown through her? Being insensitive to pain is one thing but she has a massive gap in her spine, is the tissue of the sides of a person's abdomen strong enough to support that much weight on it's own?
The gap's not as massive as it looks in the poster used as the main page's image, but it is pretty big. Probably best to assume that since the whole reason she's alive and mobile at all after acquiring said hole (besides the fact that the movie would be a little difficult to stage if she spent the rest of it crawling around on her hands dragging her lower half behind her) is a magic potion, essentially A Wizard Did It for real.
To add to that, what about non-fatal injuries? Are you pretty much screwed if you chopped off a finger by mistake, or would that grow back at least? Lisle's potion seems to heal after she demonstrates by cutting her clients' fingers, so it seems a little odd that a mere broken neck wouldn't heal up as well.
Why didn't a scientifically minded person try to perfect the potion? Because it's magic. Why hasn't Lisle herself or some other magician tried to include a healing factor? Because that would be boring (or it would be an entirely different movie). If there's no healing factor, why did it heal Mad's finger when Lisle dabbed a drop on her cut? It seems like the potion "healed" Mad of her age when she drank it; her boobs and butt get higher, her hands and face smooth out, etc. I would imagine that when drunk, the potion "heals" the drinker to their prime physical state, but then locks them there. It healed the cut on Mad's finger because she hadn't yet drunk it; and Lisle only put a drop onto the cut, not enough to make Mad completely immortal. Alternately, it's plausible that the potion simply keeps you alive and human forever, with standard human healing. In that case, you could heal simple cuts and scrapes, but fatal injuries would turn you into a zombie.
Do potion-drinkers become immune to crippling illnesses? For example, many vaccines must be renewed every so often because of mutations in the viruses and related factors. Imagine living forever but getting screwed over due to polio or cancer or something.