The answer to 'how much to fix backflow preventer' is that the cost can vary significantly depending on several factors. Backflow preventer repair or replacement is typically a job for a professional plumber, and the pricing will depend on the specific issue, the type of backflow preventer, the location and accessibility, and the labor rates in your local area.
Generally, you can expect to pay anywhere from $150 to $500 or more to have a backflow preventer serviced by a professional plumber. More complex issues or the need to replace the entire unit can push the cost higher. It's important to get local quotes to understand the specific costs in your area.