Airdrops should be free but it can come with some tasks which you do not require anyone to pay any money.
But I also saw some airdrops asking people to pay them little amounts of TON or Tron, in order to put them in a better place in a leaderboard.
This is how scam was in airdrops in the past. Many will become scam. Do not pay.
It's actually easy if you believe and want to get maximum results, then if there is a payment, do it, but if you are satisfied with the results without paying, that's enough ,
because I noticed that the average airdrop ton all made transactions of 0.2 tons if you do a lot of airdrops you need a lot of tons so it's all based on the level of trust and token supply I think