Taxis are cheap in Sofia and it's only 12-16 leva (6-8 euro) in a metered taxi from the airport to the hilton. The key is to use ok taxis which have a contract with the airport and don't (most of the time) rip you off. You can either go to the ok taxis desk at the airport who will give you the cabs number or just go outside where the taxis are waiting and just make sure you get in one of their cabs. Id definitely go for this option over the subway which involves a change plus a walk too.