You have to know that 40 km is in very heavy traffic that can take 90 minutes. An then a return trip as no pickup allowed at the airport. So 3 hours time and fuel for 800 Baht. Years ago my wife would taxi guests to airport for 1,000 Baht. These days with traffic she does not offer airport transfer, indeed I just tell even friends just to take the taxi when arrive at Phuket airport.