Wot - that will also depend. MOST (not all) will charge by your LOD. Independents will charge less than your "companies". When I was in Channel Islands California
, I used a 'company' - $1.35/ft x 43' (LOA) plus $10 per zinc to replace (with mine) or plus the cost of zincs.
Here in Ventura I use an independent, $1/ft LWL free zinc replacement with your (my) zinc.