Anyone know how expensive it is to reset a diamond?

no idea, didn't have one to reset.

Have you tried using the blunt end of a spoon or knife to ease the post back against the stone? If that isnt enough, have a chat to a jeweller and get a quote before committing to having it reset. Different types of mounts are set in different ways, most are quite simple but he'll want to check the metal hasnt been fractured which could affect the cost.

Take it back to your jeweler and they will most often do it for free. Ive had mine reset in my wedding ring on 2 differnt occasions and they did it for free.

It also depends on the size of the diamond too, and the ring. Take care - DNatureofDTrain