Addition to cost (9)
I believe the answer is:
surcharge
<span class="explanation-format__ClueExplanation">'<span class="explanation-format__original">addition to cost</span>' is the definition.<br/ ><span class="explanation-format__explanation">(I've seen this before)</span><br/ ><br/ >This is the entire clue.<br/ ></span>
(Other definitions for surcharge that I've seen before include "Extra payment" , "Tax" , "Additional sum demanded" , "Addition to usual payment" , "supplement" .)