Fair enough. Though I don't think Ashwin and Kumble had that huge of a difference..... I generally put more importance in WPM than others for spinners; as it shows their importance in the team. I think in general it's expected for Spinners to be more expensive while producing breakthroughs when needed. Spinners like Kumble, Qadir, Gibbs, Vettori, Mushtaq, Swann, etc. have really not an average to boast for but have really good WPM. Though, I won't blame anyone for putting more value in average; how much a bowler costs for each wicket they take is really important also. I think a difficult group of Indian spinners to judge are the spin quarter of India. They don't have much good averages; but what do you expect having 4 spinners and at times no pacers in your team; in pitches like Australia and England.
And for context; I only put 3 spinners (Warne, Murli, O'Reilly) among the highest tier of bowlers and four others (Ashwin, Grimmett, Kumble, Laker) in a tier below.