I think it's more a case of they have reason to doubt da costa, wheras they are still unsure about Kvyat. Also, Russia is a big market for Red Bull. They have 2 teams in F1 and the cost of running a F1 team will be increased for both of their teams next season, so if they can balance that by taking extra finance then it's a positive reason for sure. Surely if he competes reasonably with Vergne, he shows he has good potential to improve and become someone who could take over from Vettel if he moves on, if he doesn't then they will have to buy someone in. da Costa might be a safer bet for a decent F1 driver, but perhaps not for a F1 champion.
Have to feel for da Costa though, if he came along 1 year later, then the form he showed last year would be happening now, and he would have got the drive easily. People would be talking whether he should be driving for Red Bull, not Toro Rosso...not whether he should be in F1.