The tank itself or tank and labor? A ball park estimate for tanks is about $10 per gallon.( shipping can add a lot to that number if needed) The labor that can add up.
Labor costs are affected by several factors: How hard or easy it is to get to and pull the tank ( access panel or need to cut the deck), condition of the tank coffin (need to repair rot etc), labor and parts to replace fill and vent plumbing ( if you are going to replace the tank, you should replace all hoses , sender, etc.