It definitely depends of the coin which is doing airdrops, There are some which give you free coins but he effort is huge for a minimum amount, and it is not sure it will be profitable.

Me for example, I am on DeepOnion, and there are 40 weeks for airdrop and i just joined and will get my first airdrop, but the coin has a lot of future and the price will increase once the airdrop if finished. So in this case the rules are reachable and you get a good amount of coins, there are still 9 weeks for airdrops and the community is increasing a lot every week. So that is one profitable aidrop that I may recommend to you.