Typically, I don't really get "calls" for this problem. I usually discover the problem while performing other work, like installing a new fixture or ceiling fan or a device change-out.
The epoxy works well, although sometimes getting it to stick to an old box requires some clean-up on the old box, such as course filing or sanding and blowing it out to give the epoxy a foothold. If it's a box that's accessible from the attic, i.e. for a ceiling fan, I'll change it out with a fan hanger and a metal box.
As for the charge, handle it like any other service:
Inspect and come up with a plan
Show it to the client and explain the work required
Estimate the cost and receive approval (CHANGE ORDER)
Perform the work on a T & M basis, never charging more than estimated
Of course, if it broke when you touched it, you're probably working for free for about a half hour or more... Nature of the business...
Good Luck!
