Additionally, you should add the 145,000 hits for incentivise to the 242,000 hits of incentivize for 387,000.

And I'd like to subtract the ~10,000 hits relating to a product, Xactly Incent even though it's somewhat apropos. (I generally like to eliminate popular songs, movies, band names, etc. from this type of comparison as much as possible)

Another oddity...
incents only gets 34,900 hits. On the first 3 pages, 90% are misspellings for the smelly stuff (and I'm guessing the anger and the imbue with scent might also be represented).
incentivises 35,400 + incentivizes 30,700 = 66,100 and is unlikely to have been confused with another word
... so incents vs incentiviz(s)es is "estimated 35" vs 66,000?