Generally airdrops can be small amounts  for example something like 5 dollar or 10 dollar. But it is very good in my opinion because that is increasing awareness about project if it will be even 5 dollar. Because people can earn 5 dollar only with giving some eth adress. That is not bad and also that is important for projects. But generally bounties are more profitable than airdrops.  For example pchain did some airdrop for their telegram channel and they have a telegram channel with more than 45k people and i think they did that only with 1 dollar or 2 dollar value of airdrop per person.