I think it's hard to measure spinners in that sense. A lot of those mentioned are freaks, in a certain way. If you look historically at how spinners have operated and their stats, a lot of those mentioned are exceptions. Vettori is a funny one, because he averaged closed to 40 for ages (and if you remove Bangladesh & Zimbabwe as people like to do, his overall stats are close to 40) and brought it down later on. I think the question is what do we want from a spinner when we play half our matches at home in non-spin friendly conditions, and then another slew in Australia, South Africa and England, which aren't receptive to spin either?
I'd be happy with someone who can average 35. 38-40 is realistic, but only if they go at less than 3.2 and can actually contain for the seam bowlers. Conversely, they need to be up to step their gears up when they have friendly conditions and become threatening.
So far, Craig has shown the most promise in the latter, Santner in the former.