The way it works is you pay your money which includes a core charge. If you return a core then you are given credit for the core charge you orignally paid if you don't hae a core then there's no credit.
For example: When I swapped 73-76 A-body disks onto my 68 Barracuda rebuilt calipers were $30 each plus a $30 each core charge so I paid $60 for the caliper. When I returned the dead caliper I got $30 back.
Sometimes the core charge is hardly worth the effort to return other times it's more than the rebuilt cost.