$10 to $20 - depends on the construction of the shoe and what needs to be done to steady the heel.

If the heel is totally separate from the rest of the shoe's construction, $10. If the shoe has to be partially rebuilt (often the case when you are dealing with a wedge or other designed heel), I would expect $20 or more.

FWIW, I broke the heel on a favorite pair of custom slingbacks I had made in Hong Kong and because the heel separated from the construction point on the shoe, I'm going to take it back to HKG to be repaired rather than have a local store do it since part of the shoe will need to be rebuilt.