The highest I have seen is in the low 80's and for my own account in the low 50's.
Although I think it's just a minor element in your overall hubpages success.
I mainly gauge my hubs success by search engine traffic and search ranking with minimal promoting and backlinking.
All the numbers change from week to week anyway, just like traffic. Although if you have a karma score of 50 or above your doing okay and your hub won't be seen by Hubapges as spammy.
My feelings are if you chase Karma score too much you might lose link juice and your search result traffic may suffer, so I guess everyone walks a narrow line between keeping Hubpages happy and having your Hubs effective in SERP's.
For me I mainly concentrate on improving my writing and looking for topics of greatest value for readers, this philosophy has taken my hubs from strength to strength and given me ever increasing traffic and of course revenue.