I've read (I have no sources) that while the "zerodium still pays more for an android 0-day" thing can be true, the conditions on the "top" payout are pretty strict, due to the same aforementioned variability between vendors. To get that payout you'd have to find something exploitable on nearly all vendors version of android along with working on 2-3 versions. In reality an iMessage exploit is going to pay out a lot more because it would be exploitable on nearly all iPhones running x version of iOS, for example. Finding an exploit in say "Samsung messenger" (I don't know if that even exists) would pay less than an equivalent iMessage one.