After you have a detailed cost estimate, triple it to account for inevitable cost overruns due to various "unanticipated" reasons, which always materialize, eg

After you have a detailed cost estimate, triple it to account for inevitable cost overruns due to various "unanticipated" reasons, which always materialize, eg: tooling error, parts out of stock, incorrect tolerance, Chinese new year, or manufacturer just doesn't deliver. Quadruple that number again to estimate your retail price.