@CoL - That chance is only 5%. Hardly worth mentioning in fact.
If said merchant's guild (or whatever) can affor the poison and a thug, chances are that a low level blackguard or assassin would be within their means. No poison fumbles there.
I am a strawman, and I approve this message.
Ok. Moving the goal posts. Before it was an Expert and a Barmaid (presumably a Commoner or Expert). Had you set this from the start it would not have been an issue (and said low level blackguard or assassin is at least level 6 and could just ya know, solo the level 5 wizard and save the time and trouble but hey). It is a CR + 5 encounter (at least) after all since the mage is alone. Casters are awesome, but not that awesome. They'll die if you powertrip enough (which is what expensive consumables and CR + 5 or more encounters are and can only be). Rocks fall, everyone dies proves nothing.
For starters, "I" didn't set anything up. I was simply offering an opinion ased upon the rules of poison use. Only amateurs use NPC throw-aways to assassinate people. Besides, they didn't have goalposts back then.
Secondly, planning out an assassination is not power tripping. Surely, there must be SOME way of occasionally knocking one off without resorting to dropping rocks on the tavern? As to the "+5 CR" thing, HUH? We are talking about a lvl. 1 wizard? It makes my brain hurt that you would spens so much time debating the best way to assassinate a 1st level wizard. Try this!
Just get a sack full of common cats, shake it up, and toss it in the room. Odds are, just their yowling will be enough to reduce the 6-10hp said mighty wizard has. Done deal.
No expensive poison, no expensive blackguard, just cats.