I'd probably be willing to drop $600 on a iPad Pro Mini, provided it actually had the Pro features, edge to edge screen, upgraded internals, A12x, etc... It's actually not that far off what I paid for my Mini 2 originally. Heck, given the Pro's features, I'd probably even go as high as $800. I love the form factor that much to be willing to pay those kinds of prices.
But if they just take a Mini 3, throw a 3 year old A10 in it, and try to sell it for $600, no way. Even at $299 it'd be a hard sell, because it's not really enough of a leap from my existing Mini 2. That's just planned obsolescence starting it off already outdated.
Basically, if they keep looking at the Mini as an 'entry level' device only, and constantly give it underpowered, outdated hardware, then I'll just keep using my old one until it completely dies. If they get serious about it, and recognize it for the portable powerhouse it could be, I'm on board.