I can't see him going. Maybe in January.
The way I see it, I can't see anyone paying the release fee, apart from Chelsea if Costa gets his (rumoured) way and returns to Spain.
Kismet, in that prank call, stated CB would do anything legally to leave Villa for a bigger club, but lets not forget Kismet was the one who agreed to a £32.5m release clause in his contract. This would surely be the "legal" release from contract and is only relevant if a suitor is willing to pay it.
I can see him going in January for either his release clause if he starts the season on fire, or around £15-20m if he has an average start.