My take is: The higher the price and the lower the gain, the easier it is to reconcile with canon (because it will never come up except in the direst of circumstances). I'll suggest that you get a number of pawns equal to the magus' might in corpus (corpus score/5). So only magi and possibly apprentices, no ungifted people or hedge magi.
Another question is whether you can get back the corpus vis used to heal someone. I'll suggest you can, until the wound would have healed naturally. It has to fade, otherwise you'll get some incredible corpus reservoirs.
Personally, I do not think it should work as a rule, for the sake of the story. However, I can see many interesting, exceptional stories, where it might work as an exception.
My favourite would be something like leper magic, where the magus can draw vis from somebody else by inflicting leprosy (or another kind of horrible wound). This should require a live victim though.
I could imagine a saga where the Gift leaves vis in the corpse, just like Might does. I would prefer just a flat (say) 4p per corpse. Having it dependent on some art score feels arbitrary, and raises all sorts of issues with Gifted hedge and non-wizards. One could have a similar 1p per minor supernatural virtue and 3p per major, in a vis-rich (and cut-throat) saga. As I may have suggested before, if it's worth doing, it is worth over-doing, so TBH I would be quite curious to play that cut-throat variant, even if it may feel a little White Wolf.
I'm pretty sure this exists, because I'm sure I've read a story seed somewhere about a Bonisagus coming around to adopt people's apprentices, and these apprentices keep dieing. People suspect the Bonisagus needs corpus vis from the dead apprentices to stay alive.
I'm also having a vague flashback to older editions and the Tomb of the Archmagi, where the eldest magi are laid to rest. The tombs are kept warded - partly to stop people robbing arcane connections to ghosts, partly to stop people trying rob magical grave goods. Was there a possibility of people trying to get vis from corpses or ghosts there?
You might be remembering a Quaesitor case from Hoh:TL. (She's found innocent, but dies in one of seven simultaneous Wizard Wars soon afterwards.) There's no mention that she was getting Vis out of them: the apparent reason is that she was just reckless with their lives, perhaps at the level of "I'll try to kill you, let's see if my defensive spell saves you". But no details are given.
I am not convinced that all magi should be considered as magical creatures. I think that the Gift is not a capacity to contain vis, but that it is a capacity to manipulate vis.
However, a magi with some other virtues or flaws than The Gift (Faerie blood, Giant Blood, Mythic Blood, Lycanthropy might be good example) could be considered as magical creatures.