Realistically there's not a rental around this year that would cost Caissie, and it's not even close. It would take a really extreme example for a hitter to cost that much, someone like Tucker coming available. SPs cost a premium at the deadline so it wouldn't take as extreme of an example, but I still don't see it among current options. Dylan Cease, Michael King, and Framber Valdez are the guys who might warrant it on talent, but their teams aren't selling.
When we traded for Jeimer Candelario at the deadline two years ago he was the best rental bat moved, and he cost the Cubs' 14th and 16th best prospects. The equivalent today would probably be something like Will Sanders and Pedro Ramirez. Christian Franklin and Ryan Gallagher would be another comparable package.
Caissie seems very likely to get moved, but it's almost surely going to be for someone under control through at least next year.