I'm afraid the times when you could make a lot of money on airdrops are over and I doubt they will come back. Developers understood that airdrops no longer have as much marketing power as they used to be, and the only thing that happens is the drop of price right after the token is listed on exchange. Therefore, if they are giving away tokens for free, they are very small amounts. The only way to earn money is to forget about them for a few years and if you are lucky and the project is successful, then you can earn.