40 mana is the correct cost.
With that said, the damage it does is incorrect as well.
The spell had an overhaul at some point.
Classic Tainted Breath Conj 40m 10/8/7 Poison DoT Vendor - EI
10/8/7 means 10 damage on impact and 8 damage per tick for 7 ticks.
http://web.archive.org/web/200203050...ll.asp?Id=1184
Here's a link from castersrealm's archives with the actual data.