Your idea is kind of similar to this one that
Kuroaitou made long ago (and it did really well). Searching for it took a while, and it's older than I thought.
As for your idea - it's kind of weird to say 'for each poisoned opponent,' because there's only one. So I'm going to interpret what you mean as 'for each poisoned player.' For an ability that's dynamic every turn, it seems appropriately costed, if not a bit higher than necessary. Part of the cost of this card, making it useful and making use of it, is paid for by getting poison out there in the first place. Sure, the opponent could help you, but that's unreliable. Also, typically an upgrade bonus of +1/+1 doesn't necessitate a cost increase.
If I'm understanding your idea right, for your current text, the correct phrasing would be something like this: Every turn, gain one life and +1/+0 for each poisoned player and poisoned creature you control.
So if that's indeed how you intended your idea to work, then a more appropriate cost might be 6
| 6