Bowling averages of around 15 are pretty standard for club cricket, especially T20 stuff.
Now I will start by saying I am not trying to be argumentative at all.
Iv just looked through Play cricket and we don’t have a single bowler (who has taken more than 25 wickets) with an average at 12’s or even that close.
As a one of season that would be impressive, relevant to the level of cricket you play but if you or anyone achieves a consistent return of a wicket every 12 runs they are playing well below the level they should be playing.
Our top 4 front line bowlers are all between 20 & 35 and that’s across there playing careers which seems to be the norm from what I can make out