This would depend on the amount of *damage* per peice obviously; but to just clean them from debris - I would not see charging more than $15.00 to $25.00 per item.
Only other thing would be the Size - it it were a very large necklace or braclet...
There are factors involved - not a simple $$ answer question!
Is it fire damage as in soot? Or mud? Or what type of damage?