Airport transfers in Dubrovnik are regulated by the Dubrovnik Taxi Authority and a price range is fixed for most destinations. You can see the price list as posted at the airport in the photo below:
These prices are based upon a car that generally seats up to 3 people with luggage. If you require a van or minibus prices will be higher. You may find some agencies or individuals online who offer transfers for cheaper rates, but many of them are unlicensed and it is illegal. Non-licensed drivers often undercut legal taxi drivers and take business away from them. If you want to assure that your transfer is done safely and by a licensed driver, be sure to only book transfers with a legal taxi company.