Not being there, that would be only a guess.
I think $300 would have been way low!
I've seen salt in toilet lines before. Those are customers that tend to rarely flush the toilets to conserve water usage. In the Northwest, we don't get calcium deposits in the pipes. What are others doing for that?
If it's salt, I would consider using some Bio-Clean on it.Muriatic acid will clean out the calcium build up.